[videoinfoscanner] do database clean off main thread

Moves it to VideoInfoScanner and changes the modal progress dialog to background one. Fixes freezing issues discussed here: http://forum.kodi.tv/showthread.php?tid=207697  (**Note: does not fix for music.** That is still done in Builtin.cpp)

@mkortstiege if you have time. I'm not going to lie to you: it's not pretty.