To support connecting to MPD instances on other hosts, or if the socket
is in a different location, the host and/or port can be specified using
either command-line arguments or environment variables. The behavior is
similar to that of `mpc` in that the password can be specified in the
host value using `password@host` syntax. The command-line arguments
override the environment variables. If neither are specified, the
default of `localhost:6600` is used.
This implementation does not include key bindings or a status icon,
because I wanted to keep it small, and both of those require linking
with and initializing GTK+.
Older versions of GObject had the "user data" parameter of most signal
handlers marked as required, some value had to be passed in, even if it
wasn't necessary.