Use ConfigBinding in Channel.

Not sure why we didn't do this before.

Also unregister the config listener when the channel is closed,
because not leaking things is Good.

Change-Id: I76aaf1f9dad22c2066f78d66a0d87da24320e2a0
Reviewed-on: http://gerrit.dmdirc.com/3638
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>