RegionFactory::CheckNewRegion signal is emitted directly
after region-creation from RegionFactory::create().
At this point in time the region is not on any playlist.
This if fine for EditorSources, but the new RegionList design
only shows regions that are on the timeline.
CheckNewRegion() -> EditorRegions::add_region()
-> EditorRegions::regions_changed() ignores regions w/o playlist.
Later Playlist::add_region_internal() sets the playlist but
calls Region::clear_changes() to not send all individual changes.
So Region::RegionsPropertyChanged is not emitted either.
We need to notify the EditorRegions *after* setting the playlist.
The downside of this is that more signals than necessary are
emitted.
4d269729b1