app: port GimpDrawableUndo, GimpDrawable::push_undo() and ::swap_tiles()

which gets rid of the manual implementation of sparse undo buffers,
but GEGL will take care of proper COW here soo enough.

app/core/gimpdrawableundo.h

@@ -36,8 +36,7 @@ struct _GimpDrawableUndo
 {
   GimpItemUndo  parent_instance;
 
-  TileManager  *tiles;
-  gboolean      sparse;
+  GeglBuffer   *buffer;
   gint          x;
   gint          y;
   gint          width;