Replaces map_to_relative_path() from Mapper.cxx.

Prepare for the plugin interface.

Keep only a reference.

Prepare to make this a new plugin API, for example to use a SMB share
for the music_directory.

Decouples db/update/Remove.cpp from global variables.

Remove dependency on the DatabaseSimple library.

Reduce overhead.

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

Decouple command/OtherCommands.cxx from the "simple" database plugin.

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.

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.