Fixes a memory leak, or an assertion failure in the debug build.

Fixes assertion failure.

`light_song.Get()` could cause an assertion failure because the
`Manual<>` object must not be used if uninitialized.

Regression by commit ebc006ab

This function implements the "group" feature on top of the
Database::Visit() method.

Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

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.

So long, autotools!  This is my last MPD related project to migrate
away from it.  It has its strengths, but also very obvious weaknesses
and weirdnesses.  Today, many of its quirks are not needed anymore,
and are cumbersome and slow.  Now welcome our new Meson overlords!

This adds support for the full set of MPD 0.21 filter types.

That way, each plugin can decide to implement it better.

This passes the new `sort` and `window` attributes to the mounted
database.

No undefined behavior.

Requires libmpdclient 2.15.

Prepare to allow more complex expressions.

Modeled after std::filesystem::operator/() from C++17.

This enforces the "not nullptr" rule.

Prepare to make LightSong non-assignable.

Closes #284

Closes #283

Closes #258

Fixes "search already in progress" errors.