St: drop StClickable, add some functionality to StButton

For historical reasons, we had both StClickable and StButton, which
were nearly identical. StButton was more widely-used, so keep that and
port all StClickable users to that.

https://bugzilla.gnome.org/show_bug.cgi?id=640583

js/ui/altTab.js

@@ -593,8 +593,8 @@ SwitcherList.prototype = {
     },
 
     addItem : function(item) {
-        let bbox = new St.Clickable({ style_class: 'item-box',
-                                      reactive: true });
+        let bbox = new St.Button({ style_class: 'item-box',
+                                   reactive: true });
 
         bbox.set_child(item);
         this._list.add_actor(bbox);