* app/core/gimpdrawable.[ch]: add "gboolean push_undo" to
  GimpDrawable::convert_type().

* app/core/gimpdrawable-convert.[ch]: same here for the gray and rgb
  conversion functions.

* app/core/gimpchannel.c
* app/core/gimplayer.c: pass FALSE when called from GimpItem::convert()
  because it can be called on unattached items only.

* app/core/gimpimage-convert.c: pass TRUE.

* app/core/core-enums.[ch]
* app/core/gimpgrouplayerundo.[ch]
* app/core/gimpimage-undo-push.[ch]: add GIMP_UNDO_GROUP_LAYER_CONVERT
  which simply calls gimp_drawable_convert_type() with the old type
  when undone/redone.

* app/core/gimpgrouplayer.c: push a group layer convert undo so this
  can be properly undone/redone.