- get rid of the individual undo types and add GIMP_UNDO_ITEM_REORDER.
- replace the pushing functions by a single one.
- merge all the actual undo code into gimpitempropundo.[ch].
- use gimp_item_tree_reorder_item() to do the actual reordering.
- fix gimp_item_tree_reorder_item() to use an ordinary "push_undo"
  boolean again instead of a pointer to an undo function.

GimpVectorsPropUndo is now a completely empty skeleton. Keep it around
anyway, maybe we'll get vectors properties to undo soon.