elm theme - add a blank focus style if you want invisible focus

in some cases you may want a widget to focus but not to get a blue
hilightaround it - thus provide a blank focus style just for this.

@feature

legacy/elementary/data/themes/edc/elm/focus.edc

@@ -155,3 +155,8 @@ group { name: "elm/focus_highlight/top/default";
       }
    }
 }
+
+group { name: "elm/focus_highlight/top/blank";
+   parts {
+   }
+}