make selectability of a TimeAxisView be overridable

gtk2_ardour/time_axis_view.cc

@@ -510,7 +510,9 @@ TimeAxisView::controls_ebox_button_release (GdkEventButton* ev)
 
 	switch (ev->button) {
 	case 1:
-		selection_click (ev);
+		if (selectable()) {
+			selection_click (ev);
+		}
 		break;
 
 	case 3:

gtk2_ardour/time_axis_view.h

@@ -134,6 +134,7 @@ class TimeAxisView : public virtual AxisView
 	bool hidden () const { return _hidden; }
 
 	void set_selected (bool);
+	virtual bool selectable() const { return true; }
 
 	/**
 	 * potential handler for entered events