Move code from config_parse_path().

Don't use global variables.

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.

Replaces GLib's GError.

More preparations for multi-player support.

Eliminate the C wrappers.

playlist and player_control are deprecated.

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

Reduce access to the global variable "g_playlist".