Add a new CommandResult code called "FINISH" which, unlike "CLOSE",
will attempt to flush the output buffer.  This is a one-shot attempt;
it will do one write, and not try again.

This command was removed by commit 206392ad (MPD 0.16), even though it
was been proven useful for some very simple clients.  On request, I
add it to the protocol again.

Replaces GLib's g_ascii_strcasecmp().

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.

Use forward declaration instead.

Replaces GLib's GError.

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

.. instead of rolling own classes.

Eliminate the hack in handle_idle().