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

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Replaces StatFile(), with a portable data object.

The root WatchDirectory instance already contains the music directory,
and we don't need to fetch it again.

Move global variables into the new classes.  That may allow multiple
update threads for multiple databases one day.

The new class Path only holds a string pointer without being
responsible for allocation/deallocation.  The FileSystem.hxx library
accepts Path arguments instead of AllocatedPath, to avoid forcing
callers to allocate another string object.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

Let STL manage the WatchDirectory allocations.