Actually a design change: formerly we were relying on weak typing of enums,
i.e. assigning values from the wrong enum to e.g. to type 'LineJoin'. Now
*_DEFAULT variants are part of the main enum, e.g LineJoin and has to be
handled explicitely on the consumer level, here mostly renderers.

Removes the following warnings:

dia_svg.c:71:56: warning: implicit conversion from enumeration type 'enum DiaSvgLineDefaults' to different enumeration type 'LineCaps' [-Wenum-conversion]
  gs->linecap = parent_style ? parent_style->linecap : DIA_SVG_LINECAPS_DEFAULT;
              ~                                        ^~~~~~~~~~~~~~~~~~~~~~~~
dia_svg.c:72:58: warning: implicit conversion from enumeration type 'enum DiaSvgLineDefaults' to different enumeration type 'LineJoin' [-Wenum-conversion]
  gs->linejoin = parent_style ? parent_style->linejoin : DIA_SVG_LINEJOIN_DEFAULT;
               ~                                         ^~~~~~~~~~~~~~~~~~~~~~~~
dia_svg.c:73:60: warning: implicit conversion from enumeration type 'enum DiaSvgLineDefaults' to different enumeration type 'LineStyle' [-Wenum-conversion]
  gs->linestyle = parent_style ? parent_style->linestyle : DIA_SVG_LINESTYLE_DEFAULT;
                ~                                          ^~~~~~~~~~~~~~~~~~~~~~~~~
dia_svg.c:484:19: warning: implicit conversion from enumeration type 'enum DiaSvgLineDefaults' to different enumeration type 'LineJoin' [-Wenum-conversion]
    s->linejoin = DIA_SVG_LINEJOIN_DEFAULT;
                ~ ^~~~~~~~~~~~~~~~~~~~~~~~
dia_svg.c:496:18: warning: implicit conversion from enumeration type 'enum DiaSvgLineDefaults' to different enumeration type 'LineCaps' [-Wenum-conversion]
    s->linecap = DIA_SVG_LINECAPS_DEFAULT;
               ~ ^~~~~~~~~~~~~~~~~~~~~~~~
dia_svg.c:694:17: warning: implicit conversion from enumeration type 'enum DiaSvgLineDefaults' to different enumeration type 'LineStyle' [-Wenum-conversion]
        s->linestyle = DIA_SVG_LINESTYLE_DEFAULT;
                     ~ ^~~~~~~~~~~~~~~~~~~~~~~~~
shape_info.c:121:32: warning: comparison of constant -1 with expression of type 'Alignment' is always false [-Wtautological-constant-out-of-range-compare]
      if (el->text.s.alignment == -1)
          ~~~~~~~~~~~~~~~~~~~~ ^  ~~
shape_info.c:911:2: warning: implicit conversion from enumeration type 'enum DiaSvgLineDefaults' to different enumeration type 'LineCaps' [-Wenum-conversion]
        DIA_SVG_LINECAPS_DEFAULT, DIA_SVG_LINEJOIN_DEFAULT, DIA_SVG_LINESTYLE_DEFAULT, 1.0
        ^~~~~~~~~~~~~~~~~~~~~~~~
shape_info.c:911:28: warning: implicit conversion from enumeration type 'enum DiaSvgLineDefaults' to different enumeration type 'LineJoin' [-Wenum-conversion]
        DIA_SVG_LINECAPS_DEFAULT, DIA_SVG_LINEJOIN_DEFAULT, DIA_SVG_LINESTYLE_DEFAULT, 1.0
                                  ^~~~~~~~~~~~~~~~~~~~~~~~
shape_info.c:911:54: warning: implicit conversion from enumeration type 'enum DiaSvgLineDefaults' to different enumeration type 'LineStyle' [-Wenum-conversion]
        DIA_SVG_LINECAPS_DEFAULT, DIA_SVG_LINEJOIN_DEFAULT, DIA_SVG_LINESTYLE_DEFAULT, 1.0
                                                            ^~~~~~~~~~~~~~~~~~~~~~~~~
custom_object.c:926:26: warning: comparison of constant 'DIA_SVG_LINECAPS_DEFAULT' (20) with expression of type 'LineCaps' is always false [-Wtautological-constant-out-of-range-compare]
  if ((el->any.s.linecap == DIA_SVG_LINECAPS_DEFAULT && (*cur_caps) != LINECAPS_BUTT) ||
       ~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~
custom_object.c:928:39: warning: comparison of constant 'DIA_SVG_LINECAPS_DEFAULT' (20) with expression of type 'LineCaps' is always true [-Wtautological-constant-out-of-range-compare]
      (*cur_caps) = (el->any.s.linecap!=DIA_SVG_LINECAPS_DEFAULT) ?
                     ~~~~~~~~~~~~~~~~~^ ~~~~~~~~~~~~~~~~~~~~~~~~
custom_object.c:932:26: warning: comparison of constant 'DIA_SVG_LINEJOIN_DEFAULT' (20) with expression of type 'LineJoin' is always false [-Wtautological-constant-out-of-range-compare]
  if ((el->any.s.linejoin==DIA_SVG_LINEJOIN_DEFAULT && (*cur_join)!=LINEJOIN_MITER) ||
       ~~~~~~~~~~~~~~~~~~^ ~~~~~~~~~~~~~~~~~~~~~~~~
custom_object.c:934:38: warning: comparison of constant 'DIA_SVG_LINEJOIN_DEFAULT' (20) with expression of type 'LineJoin' is always true [-Wtautological-constant-out-of-range-compare]
    (*cur_join) = (el->any.s.linejoin!=DIA_SVG_LINEJOIN_DEFAULT) ?
                   ~~~~~~~~~~~~~~~~~~^ ~~~~~~~~~~~~~~~~~~~~~~~~
custom_object.c:938:28: warning: comparison of constant 'DIA_SVG_LINESTYLE_DEFAULT' (20) with expression of type 'LineStyle' is always false [-Wtautological-constant-out-of-range-compare]
  if ((el->any.s.linestyle == DIA_SVG_LINESTYLE_DEFAULT &&
       ~~~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~~
custom_object.c:940:40: warning: comparison of constant 'DIA_SVG_LINESTYLE_DEFAULT' (20) with expression of type 'LineStyle' is always true [-Wtautological-constant-out-of-range-compare]
    (*cur_style) = (el->any.s.linestyle!=DIA_SVG_LINESTYLE_DEFAULT) ?
                    ~~~~~~~~~~~~~~~~~~~^ ~~~~~~~~~~~~~~~~~~~~~~~~~
svg-import.c:468:25: warning: comparison of constant 'DIA_SVG_LINESTYLE_DEFAULT' (20) with expression of type 'LineStyle' is always true [-Wtautological-constant-out-of-range-compare]
      if (gs->linestyle != DIA_SVG_LINESTYLE_DEFAULT)
          ~~~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~~
svg-import.c:524:24: warning: comparison of constant 'DIA_SVG_LINEJOIN_DEFAULT' (20) with expression of type 'LineJoin' is always true [-Wtautological-constant-out-of-range-compare]
      if (gs->linejoin != DIA_SVG_LINEJOIN_DEFAULT)
          ~~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~
svg-import.c:530:23: warning: comparison of constant 'DIA_SVG_LINECAPS_DEFAULT' (20) with expression of type 'LineCaps' is always true [-Wtautological-constant-out-of-range-compare]
      if (gs->linecap != DIA_SVG_LINECAPS_DEFAULT)
          ~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~