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.

rtdata/themes/size.css

@@ -351,6 +351,11 @@ menu arrow {
     margin: 0 -0.25em 0 0;
 }
 
+menu menuitem > radio + * image:not(.dummy),
+#MyExpander menu menuitem > radio + * image:not(.dummy) {
+    margin-left: 1pt;
+}
+
 menu image:not(.dummy),
 #MyExpander menu image:not(.dummy) {
     min-height: 2em;