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/ctrlAltTab.js

@@ -140,9 +140,10 @@ const CtrlAltTabPopup = new Lang.Class({
     Name: 'CtrlAltTabPopup',
     Extends: SwitcherPopup.SwitcherPopup,
 
-    _createSwitcher: function() {
+    _init: function(items) {
+        this.parent(items);
+
         this._switcherList = new CtrlAltTabSwitcher(this._items);
-        return true;
     },
 
     _initialSelection: function(backward, binding) {