switcherPopup: Move _createSwitcher implementations into constructors

We don't really need this step as a separate method since all
implementations are supposed to be created and shown immediately. This
also ensures that we have items to show in all subclasses.

https://bugzilla.gnome.org/show_bug.cgi?id=735976

js/ui/status/keyboard.js

@@ -265,11 +265,8 @@ const InputSourcePopup = new Lang.Class({
 
         this._action = action;
         this._actionBackward = actionBackward;
-    },
 
-    _createSwitcher: function() {
         this._switcherList = new InputSourceSwitcher(this._items);
-        return true;
     },
 
     _initialSelection: function(backward, binding) {