The new class is pluggable, to prepare for gzipped database files.

For now, the TextFile class remains, and will be refactored away
later.

Use Storage::MapUTF8() internally, don't use global variables.

Move from db/DatabaseGlue.cxx, eliminating global variable.

Fixes loading database songs from state file (regression by commit
29072797).

Another big chunk of code for multi-player support.

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.

Migrate all callers to use Path directly, instead of doing the
conversion in each caller.

No caller needs this.  Fixes use-after-free after returning from
Client::OnTimeout().

Save the state file 2 minutes after the last change.  This reduces the
disruptions by an idle MPD, and MPD can be paged out permanently until
it is used.

This is the beginning of multi-player support.  There will be support
for multiple Partition objects in one MPD process.