Hide progress bar for library clean/scan by default if automatically triggered.

This adds {music,video}library.backgroundclean, making it consistent with library scans ({music,video}library.backgroundupdate).

Add-ons may automatically clean libraries (service.watchdog) while the user is in front of it, just waiting for it to complete to watch something. The user enables this setting and they're happy again.
My libraries aren't that big, but I suspect it may take a while (more than a couple seconds) on very huge libraries, so I think it makes sense to hide it like we do for scans.

(It may also happen that you remotely clean the video library while your mother is watching her soap opera. She'll complain, a lot.)

(I'd suggest ?w=1 to only show the relevant changes in MusicDatabase.cpp.)
showgui is not a proper name. showdialogs is more appropriate as they all are basically dialogs