fix mixed smartplaylists

These commits fix some aspects of mixed smartplaylists. The main problem is that we show the user a list of properties to filter by which are not available for both songs and musicvideos which results in very odd SQL queries and in the end very odd listings when the user tries to filter by a property that is either not available for songs or musicvideos. Therefore I have limited the available properties for filtering to the ones that are available for both songs and musicvideos. Furthermore I have fixed some "Browse" listings in case of a mixed playlist.

This takes care of http://trac.xbmc.org/ticket/14982.