Add top-level-category to config page

This allows a site using rfa.kaltura to restrict to a single category on the KMC.   For example, you can create a category called "PloneSite" on the KMC, and configure rfa.kaltura to use the "Top Level Category" of "PloneSite". Only sub-categories and videos of the "PloneSite" category will be available as choices in kaltura media edit pages.