Change single_track and shuffle exclusive
Make single_track and shuffle exclusive of each other.

Merge remote-tracking branch 'kyouryuukunn/musicroom' into maintenance/6.17
Documentation updates.
Safely list directories outside the Ren'Py base directory.

Directories in the projects directory might contain filenames
that aren't representable in the system encoding. We now
ignore these directories, rather than throwing an exception.
Merge branch 'maintenance/6.17'