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).

This enforces the "not nullptr" rule.

Prepare to make LightSong non-assignable.

Closes #284

Closes #283

Closes #258

Fixes "search already in progress" errors.