playlist: Send "current-removed" when clearing playlist

Otherwise the video widget isn't cleared, the playing property
notify signal isn't sent, etc.
main: Clear the playlist when going back from player

So has to avoid network transfers in an unreachable portion
of the UI.
grilo: Also look up saved metadata

Using the metadata store. This would allow us to cache locally
metadata from online sources such as tmdb.
screensaver: Remove lock-screensaver-on-audio preference

Totem isn't built to play audio files, so we'll always inhibit
the screensaver when playing anything. If you're playing audio
files, at least you know what you're in for.
screensaver: Wait 5 seconds before uninhibiting screensaver

Otherwise the screensaver might come on in between 2 movies.

See https://bugzilla.gnome.org/show_bug.cgi?id=705942#c7