release v0.20.22

Closes #373

The flag `-stdlib=libc++` doesn't appear to work because it attempts
to load `libc++` and not `libc++_static`, and it omits `libc++abi`.

Closes #360

Closes #366

In this new order, a filter 'AlbumArtist ""' matches only on songs
which neither have `AlbumArtist` nor `Artist`.

Grouping in the "list" command was completely broken from the start,
unlike "count group".  I have no idea what I have been thinking when I
wrote commit ae178c77, but it didn't
make any sense.

This commit is a rewrite of the feature.

For clients to be able to detect this feature, this commit also
increments the protocol version.

Be consistent with "list" responses.

Just like AlbumArtist falls back to Artist, AlbumArtistSort should
fall back tom AlbumArtist, ArtistSort and finally Artist.

Closes #355

This improves the workaround from commit
b5ba94f1 and actually gives a useful
result for "list" with a disabled tag.

Use `const_cast` to be able to pass it to sf_open_virtual() which has
a wrong parameter declaration.

Fix links and lots of broken markup.