There was quite a lot of duplicate code for loading DetachedSong
objects, with different semantics for "securely" loading local files.

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".