Change GUI to support multiple external editors

Replace radio selector in external editor section of preferences with
external editor preferences widget. Replace send-to-GIMP button with
pop-up button for exporting to a selectable application.

rtgui/popupcommon.cc

@@ -251,7 +251,7 @@ void PopUpCommon::setButtonHint()
         auto item = dynamic_cast<MyImageMenuItem*>(widget);
 
         if (item) {
-            hint += item->getLabel ()->get_text ();
+            hint += escapeHtmlChars(item->getLabel()->get_text());
         }
     }