Depsgraph/Python: callbacks and properties to detect datablock changes

* Adds two new python handlers: scene_update_pre() and scene_update_post()
  These run before and after Blender does a scene update on making modifications
  to the scene.
* Datablocks now have an is_updated property. This will be set to true in the
  above callbacks if the datablock was tagged to be updated. This works for the
  most common datablocks used for rendering: object, material, world, lamsp,
  texture, mesh, curve.
* Datablock collections also have an is_updated property. If this is set, it
  means one datablock of this type was added, removed or modified. It's also
  useful as a quick check to avoid looping over all datablocks.
* RenderEngine.view_update() can also check these properties, for interactive
  viewport rendering.

http://wiki.blender.org/index.php/Dev:2.6/Source/Render/UpdateAPI

source/blender/python/intern/bpy_app_handlers.c

@@ -47,6 +47,8 @@ static PyStructSequence_Field app_cb_info_fields[]= {
 	{(char *)"load_post", NULL},
 	{(char *)"save_pre", NULL},
 	{(char *)"save_post", NULL},
+	{(char *)"scene_update_pre", NULL},
+	{(char *)"scene_update_post", NULL},
 	{NULL}
 };