* app/core/gimpgrouplayer.[ch]: add gimp_group_layer_suspend_resize()
  and gimp_group_layer_resume_resize() and call them around functions
  where all a group's children are transformed (translated, resized
  etc).  This way we go from the worst case of reallocating the
  group's projection tiles once for each child down to exactly one
  reallocation.

* app/core/Makefile.am
* app/core/core-enums.[ch]
* app/core/core-types.h
* app/core/gimpimage-undo-push.[ch]
* app/core/gimpgrouplayerundo.[ch]: add new undo class
  GimpGroupLayerUndo which implements undos for suspend/resume of
  group layers and calls them in reverse order when undoing.