This is an initial work-around for signal emission with
Playlist:region_lock held:
Playlist::duplicate() takes RegionWriteLock() and then calls
RegionFactory::create(). The newly created region does NOT have
property-changes suspended, and the RegionFactory can change
region-properties which results in signal-emission.
This in turn can lead to a call Editor::session_gui_extents()
-> ARDOUR::Playlist::get_extent() -> Playlist::RegionReadLock
which deadlocks: https://pastebin.com/84rSbsA3
Eventually we need a mechanism to create regions with
->suspend_property_changes() and add it to the playlist's
thawlist (pass a thawlist to the region-factory).
This GTK button style is out of place in both contexts.
Furthermore it is inconsistent to not have the same button
in the editor (there is a dropzone).
