query: Only show songs from Music and Downloads directory

Filter out songs if they are not a descendant of the user's Music
or Downloads directory.

Note: The URLs of the Music and Downloads directory should be in double
quotes since URLs allow single quotes, but since it is uncommon for
Music and Downloads directory to have a single quote in path, and
tracker:uri-is-descendant do not accept arguments enclosed in double
quotes, use single quotes instead.

https://bugzilla.gnome.org/show_bug.cgi?id=706948