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.
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