Add setting to hide library clean progress bar.

This adds {music,video}library.backgroundclean, similar to backgroundupdate.
Hides the progress bar for library cleans.
json-rpc: explicitly pass showProgress in VideoLibrary.Clean.

This allows it to respect videolibrary.backgroundclean.
This way one can remotely clean the library and it won't prompt the user for
unavailable sources.
See:
https://github.com/xbmc/xbmc/blob/master/xbmc/video/VideoDatabase.cpp#L8035-L8037
https://github.com/xbmc/xbmc/blob/master/xbmc/video/VideoDatabase.cpp#L8310-L8312