allow internal generators to make noise, even if we are not rolling, and using auto-input
add mutex/lock to all Evoral::SMF methods that use _smf/libsmf, to avoid inadvertent corruption via multithreaded access.

Serialization of Session::save_state() will already protect against most of this, but there is really no
good reason why Evoral::SMF's API should require single-threaded/explicit serialization.
when flushing notifications/signals from an ARDOUR::Playlist, emit the RegionAdded/ContentsChanged signals first so that when LayersChanged is sent, the receivers know about all the new regions