speed up load/quit for sessions containing ~25k events.

- MGR uses a _note_group to hold events.

	- MRV & MGR delete canvas notes directly from their note group.

	- sysex is untested (it uses MRV note group).

gtk2_ardour/ghostregion.cc

@@ -179,6 +179,7 @@ MidiGhostRegion::MidiGhostRegion(RegionView& rv,
                                  TimeAxisView& source_tv,
                                  double initial_unit_pos)
     : GhostRegion(rv, tv.ghost_group(), tv, source_tv, initial_unit_pos)
+    , _note_group (new ArdourCanvas::Container (group))
     , _optimization_iterator(events.end())
 {
 	_outline = UIConfiguration::instance().color ("ghost track midi outline");
@@ -200,6 +201,7 @@ MidiGhostRegion::MidiGhostRegion(RegionView& rv,
                   msv.trackview(),
                   source_tv,
                   initial_unit_pos)
+    , _note_group (new ArdourCanvas::Container (group))
     , _optimization_iterator(events.end())
 {
 	_outline = UIConfiguration::instance().color ("ghost track midi outline");
@@ -324,7 +326,7 @@ MidiGhostRegion::update_contents_height ()
 void
 MidiGhostRegion::add_note (NoteBase* n)
 {
-	GhostEvent* event = new GhostEvent (n, group);
+	GhostEvent* event = new GhostEvent (n, _note_group);
 	events.insert (make_pair (n->note(), event));
 
 	event->item->set_fill_color (UIConfiguration::instance().color_mod(n->base_color(), "ghost track midi fill"));
@@ -356,12 +358,9 @@ MidiGhostRegion::add_note (NoteBase* n)
 void
 MidiGhostRegion::clear_events()
 {
-	_optimization_iterator = events.begin();
-
-	while (_optimization_iterator != events.end()) {
-		delete (*_optimization_iterator).second;
-		_optimization_iterator = events.erase (_optimization_iterator);
-	}
+	_note_group->clear (true);
+	events.clear ();
+	_optimization_iterator = events.end();
 }
 
 /** Update the x positions of our representation of a parent's note.

gtk2_ardour/ghostregion.h

@@ -116,6 +116,7 @@ public:
 	void clear_events();
 
 private:
+	ArdourCanvas::Container* _note_group;
 	ArdourCanvas::Color _outline;
 	ArdourCanvas::Rectangle* _tmp_rect;
 	ArdourCanvas::Polygon* _tmp_poly;

gtk2_ardour/midi_region_view.cc

@@ -978,10 +978,8 @@ MidiRegionView::clear_events ()
 		}
 	}
 
-	for (Events::iterator i = _events.begin(); i != _events.end(); ++i) {
-		delete *i;
-	}
 
+	_note_group->clear (true);
 	_events.clear();
 	_patch_changes.clear();
 	_sys_exes.clear();