virtualize EditingContext::midi_action() so that MidiCueEditor works

This makes MIDI bindings work in the cue editor, as intended.

gtk2_ardour/editing_context.h

@@ -341,7 +341,7 @@ public:
 	ARDOUR::Quantize* get_quantize_op ();
 	void apply_midi_note_edit_op (ARDOUR::MidiOperator& op, const RegionSelection& rs);
 	PBD::Command* apply_midi_note_edit_op_to_region (ARDOUR::MidiOperator& op, MidiView& mrv);
-	void midi_action (void (MidiView::*method)());
+	virtual void midi_action (void (MidiView::*method)());
 	std::vector<MidiView*> filter_to_unique_midi_region_views (RegionSelection const & ms) const;
 
 	void quantize_region ();

gtk2_ardour/midi_cue_editor.cc

@@ -1408,3 +1408,13 @@ MidiCueEditor::visual_changer (const VisualChange& vc)
 		//  XXX redisplay_grid (true);
 	}
 }
+
+void
+MidiCueEditor::midi_action (void (MidiView::*method)())
+{
+	if (!view) {
+		return;
+	}
+
+	(view->*method) ();
+}

gtk2_ardour/midi_cue_editor.h

@@ -94,6 +94,8 @@ class MidiCueEditor : public CueEditor
 	void maybe_autoscroll (bool, bool, bool);
 	bool autoscroll_active() const;
 
+	void midi_action (void (MidiView::*method)());
+
   protected:
 	void register_actions ();