Publish the Client API, preparing to move more code into the Client
class.

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.

Micro-optimization.

Replaces GLib's GError.

This commit also fixes incorrect passing of UTF-8 strings to client_allow_file

playlist and player_control are deprecated.

Don't use the global variable "g_playlist".

Reduce access to the global variable "g_playlist".