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;
               ~                                         ^~~~~~~~~...