mpris: Fix some typos

Wrong variables are being checked, which prevents the next lines to be
executed.

https://bugzilla.gnome.org/show_bug.cgi?id=729096
query: Filter out songs that are videos or playlists

Some nmm:MusicPiece are also a nmm:Video, and all nmm:Playlist are also
nmm:MusicPiece objects. We should filter them out because videos don't
have artist and other important fields in grilo, and playlists should
be shown in the Playlists view instead.

https://bugzilla.gnome.org/show_bug.cgi?id=726459
https://bugzilla.gnome.org/show_bug.cgi?id=728255
Added tooltips to player and header buttons

https://bugzilla.gnome.org/show_bug.cgi?id=725949
query: limit uris to xdg music and downloads folder

https://bugzilla.gnome.org/show_bug.cgi?id=706948
Release 3.12.2
Post-release version bump