Prepare for single comment-editor per route

Previously every RouteUI would create their own (Editor-mixer,
Mixer, Cue,..). Compare to StripableColorDialog.

libs/ardour/ardour/route.h

@@ -66,6 +66,7 @@
 
 class RoutePinWindowProxy;
 class PatchChangeGridDialog;
+class ArdourWindow;
 
 namespace ARDOUR {
 
@@ -138,6 +139,9 @@ public:
 	std::string comment() { return _comment; }
 	void set_comment (std::string str, void *src);
 
+	ArdourWindow* comment_editor () const { return _comment_editor_window; }
+	void set_comment_editor (ArdourWindow* w) { _comment_editor_window = w; }
+
 	bool set_name (const std::string& str);
 	static void set_name_in_state (XMLNode &, const std::string &);
 
@@ -663,6 +667,7 @@ protected:
 	std::shared_ptr<SoloSafeControl> _solo_safe_control;
 
 	std::string    _comment;
+	ArdourWindow*  _comment_editor_window;
 	bool           _have_internal_generator;
 	DataType       _default_type;
 

libs/ardour/route.cc

@@ -127,6 +127,7 @@ Route::Route (Session& sess, string name, PresentationInfo::Flag flag, DataType
 	, _pending_meter_point (MeterPostFader)
 	, _denormal_protection (false)
 	, _recordable (true)
+	, _comment_editor_window (0)
 	, _have_internal_generator (false)
 	, _default_type (default_type)
 	, _instrument_fanned_out (false)