Update Widget.getSkinName() to allow multiple skin prefixes

WF uses YUI3 to build its own framework. We are using most of YUI3 widgets but we also need to have our developers working on custom WF widgets. And to make things clear, we need to prefix our widgets skin differently than the YUI ones.

For example, we could use yui3-skin-night on a page, but we may also need to have to that same page a custom WF widget with the skin wf2-skin-toad.

Since it's only possible to change the 'yui3' prefix globally with the configuration, we took the approach of extending the capability of getSkinName: if a prefix is given, the behavior is the same as before except that the search will be made on 'prefix-skin' instead of the default 'yui3-skin'. Of course, if no prefix is given, the default remains 'yui3-skin' which ensure backward compatibility with YUI3.

Let me know if that's something that has any interest to you guys!
Thanks a lot!
Arno