Dependency Graph: some refactoring which should have no user visible impact
besides performance in some cases.

* DAG_scene_sort is now removed and replaced by DAG_relations_tag_update in
  most cases. This will clear the dependency graph, and only rebuild it right
  before it's needed again when the scene is re-evaluated.

  This is done because DAG_scene_sort is slow when called many times from
  python operators. Further the scene argument is not needed because most
  operations can potentially affect more than the current scene.

* DAG_scene_relations_update will now rebuild the dependency graph if it's not
  there yet, and DAG_scene_relations_rebuild will force a rebuild for the rare
  cases that need it.

* Remove various places where ob->recalc was set manually. This should go
  through DAG_id_tag_update() in nearly all cases instead since this is now
  a fast operation. Also removed DAG_ids_flush_update that goes along with
  such manual tagging of ob->recalc.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54727 954f8c5b-7b00-dc11-b283-0030488c597c