If we lose focus, call InstrumentTrack::silenceAllNotes() after
explicitely sending NoteOff MIDI events for all keys for which presses
were recorded. In case the arpeggiator is enabled this is not
sufficient. Fixes hanging notes when switching active subwindow while
pressing keys in an instrument track window with arpeggiator enabled.
Closes #3052228.
(cherry picked from commit 2f921303eb)