- add gimp_draw_tool_push_group()/pop_group() which manage a stack
  of groups; all items automatically get added to the stack's top group
- use push_group()/pop_group() all over the place, which saves a lot
  of code in most cases
- return GimpCanvasGroup not GimpCanvasItem pointers from
  gimp_draw_tool_add_stroke_group() and fill_group()

Unrelated:

- add GipmCanvasGroup parameter to gimp_rectangle_tool_draw()
- put rect select's round corners into the stroke group to
  avoid ugly overdrawing (the mis-alignment of arcs becomes
  very visible now however, will fix that soon)