MidiTrack::restore_controls is triggered by SessionLoaded
event. Yet MidiControl::actually_set_value checks if
_session.loading() is true and postpones sending actual
events to the synth (which may not be fully loaded).
Before 5d02970de8, Session::post_engine_init unset
"Loading" flag early on, so "loading" was already unset
by the time `SessionLoaded` was emitted.