Show filename of currently opened image in titlebar (issue 2450)

Thanks for help, Ingo!

rtgui/editwindow.h

@@ -51,6 +51,7 @@ class EditWindow : public Gtk::Window {
         bool on_delete_event(GdkEventAny* event);
         bool on_window_state_event(GdkEventWindowState* event);
         void on_mainNB_switch_page(GtkNotebookPage* page, guint page_num);
+        void set_title_decorated(Glib::ustring fname);
 
         void on_realize ();
 };