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/rtwindow.cc

@@ -1030,6 +1030,13 @@ void RTWindow::MoveFileBrowserToEditor()
     }
 }
 
+void RTWindow::updateExternalEditorWidget(int selectedIndex, const std::vector<ExternalEditor> & editors)
+{
+    if (epanel) {
+        epanel->updateExternalEditorWidget(selectedIndex, editors);
+    }
+}
+
 void RTWindow::updateProfiles (const Glib::ustring &printerProfile, rtengine::RenderingIntent printerIntent, bool printerBPC)
 {
     if (epanel) {