Replace core windowClosing impls with just close()

The base implementation of close() fires listeners then immediately
calls windowClosing(). There's no point in doing that, so we can
just override close() and free our resources after calling the super
method.

Change-Id: I87b6df3128026c77d6015f8146061aa56627e50d
Reviewed-on: http://gerrit.dmdirc.com/3011
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>