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.

Remove them as parameters from `db_selection_print()`.

May allow a better memory layout.

No undefined behavior.

Requires libmpdclient 2.15.

Prepare to allow more complex expressions.

This fixes an old bug which caused the "unused" warnings to be
unreliable; only the first block in the list was marked as being
"used", no matter if it was really used, and the rest was never marked
as "used", suppressing all warnings for them.

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

Supports .mpdignore on NFS/SMB and others (closes #290).