Adds API:
obs_scene_add_group2
obs_scene_insert_group2
obs_sceneitem_group_ungroup2

These functions should be used by plugins if they need to use these
functions and need to send a refresh signal.  If a major API rework ever
happens the old functions should be removed.  The old functions should
eventually be deprecated.

The reason why specifying a 'signal' parameter is useful is because it's
a bit more seamless for the user interface to be able to have custom
handling of these specific cases.  It looks better and doesn't require
completely erasing/recreating the entire list, which is visually
unappealing.