Fix external editor button updates

Make sure the pop-up button is updated in both multiple editors modes.

rtgui/rtwindow.h

@@ -20,6 +20,7 @@
 #include <set>
 
 #include <gtkmm.h>
+#include <sigc++/signal.h>
 
 #if defined(__APPLE__)
 #include <gtkosxapplication.h>
@@ -48,6 +49,8 @@ private:
     std::set<Glib::ustring> filesEdited;
     std::map<Glib::ustring, EditorPanel*> epanels;
 
+    sigc::signal<void> externalEditorChangedSignal;
+
     Splash* splash;
     Gtk::ProgressBar prProgBar;
     PLDBridge* pldBridge;