Add radio indicator to external editor selector

Make it easier to see that the selector is for changing the current
editor and not for immediately sending the image to the clicked-on
editor.

rtgui/editorpanel.h

@@ -246,6 +246,7 @@ private:
     Gtk::Button* queueimg;
     Gtk::Button* saveimgas;
     PopUpButton* send_to_external;
+    Gtk::RadioButtonGroup send_to_external_radio_group;
     Gtk::Button* navSync;
     Gtk::Button* navNext;
     Gtk::Button* navPrev;