Added notifier listener for node editor.

Now it behaves right on playback:

- Starting playback "Anim Player" button appears on header.
  It used to appear only on mouse hover before.
- Stopping playback triggers refresh on compositor, so
  actual result would be visible if image sequence/movie
  is used in nodes.

source/blender/editors/space_node/space_node.c

@@ -228,6 +228,13 @@ static void node_area_listener(ScrArea *sa, wmNotifier *wmn)
 			else if (wmn->action == NA_SELECTED)
 				ED_area_tag_redraw(sa);
 			break;
+		case NC_SCREEN:
+			switch(wmn->data) {
+				case ND_ANIMPLAY:
+					ED_area_tag_refresh(sa);
+					break;
+			}
+			break;
 
 		case NC_IMAGE:
 			if (wmn->action == NA_EDITED) {