ADD: use new argument for shared sections

Use the "nocat" option to display only a single "shared..." link in
linear menus (i.e. though without separate categories for movies, tv,
etc).  if option is missing, then a "shared..." section will be
displayed per section type discovered.
ADD: PleXBMC refresh for cached data

Allows cache to be deleted and recreated if new media servers have
appeared.