Improving extendibility of the sidebar improvements, css adding, and also JS variables.

Improving extendibility of the sidebar improvements, css adding, and also JS variables.

* New hooks:
** SkinSidebarSpecialBox and SkinSidebarOutputSpecialBox to allow extensions to add new custom boxes to Tim's new Sidebar system
** SkinSetupSiteCss to allow extensions to modify and add new stylesheets to load into the page. This one allows for fine positioning and can be very useful for things like an extension providing global css for a wiki farm.
** SkinGlobalVariables to allow extensions to add new global variables to export to the JS variables in the page.