As a side effect, this fixes a memory leak.

Remove the const_cast from HttpdClient.cxx, and avoid one allocation
in ClientRead.cxx.

The header glib_compat.h is now obsolete and can be removed.

Copy the according file from another project (i.e. XCSoar).  This will
allow copying more code more easily.

Try map_fs_to_utf8() first, and fall back to Path::ToUTF8() for
absolute paths.

Move code from map_fs_to_utf8().

Micro-optimization.

Consistently use std::string in LoadPlaylistFile().

Avoid the conversion to an allocated char*, let the caller decide.

Prepare to fix loading arbitrary song files from stored playlists.

Use forward declaration instead.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Un-inline the methods that use GLib.

Return Path::Null() instead of dying from assertion failure.

Fixes busy loop when USB sound device gets unplugged (Mantis bug
#3824).