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

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.