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/switcherPopup.js

@@ -102,16 +102,12 @@ const SwitcherPopup = new Lang.Class({
         this._switcherList.actor.allocate(childBox, flags);
     },
 
-    _createSwitcher: function() {
-        throw new Error('Not implemented');
-    },
-
     _initialSelection: function(backward, binding) {
         throw new Error('Not implemented');
     },
 
     show: function(backward, binding, mask) {
-        if (!this._createSwitcher())
+        if (this._items.length == 0)
             return false;
 
         if (!Main.pushModal(this.actor)) {