Reduce signal emissions from set_scale_amplitude #8576

Playlist::ContentsChanged() is implicit.

Region::send_change()
-> Playlist::region_changed ()
-> Playlist::notify_contents_changed ()
-> Playlist::ContentsChanged () /* EMIT SIGNAL*/

This cuts the number of signals in half and also
allows to freeze/thaw the playlist to collect the
signals for each playlist into a single signal.

libs/ardour/audioregion.cc

@@ -1390,14 +1390,6 @@ AudioRegion::set_scale_amplitude (gain_t g)
 
 	_scale_amplitude = g;
 
-	/* tell the diskstream we're in */
-
-	if (pl) {
-		pl->ContentsChanged();
-	}
-
-	/* tell everybody else */
-
 	send_change (PropertyChange (Properties::scale_amplitude));
 }