fix transport loop sub-state events

fix transport loop sub-state events

Loading a session with a loop-range, or setting
a loop-range adds LoopDeclick events.
Normal play back (no loop) never processes/clears the events.
Ardour was constantly doing split cycles for nothing.

This lead to stuck midi-notes: Split cycle calls
increment_global_port_buffer_offset() but the midi_diskstream
itself was not aware of any loop positions or offsets.

Entering loop playback adds those events explicitly
in Session::set_play_loop(). auto_loop_changed() only
needs to add/replace session-events when actively looping.