This will still raise an error if a window type is encountered
which can't be instansiated, but it will no longer break
horribly (e.g. refusing to allow windows to be closed)

Fixes CLIENT-166

Change-Id: Ia33d831f8dbdee0ad8a9e016587d871181ee6e42
Reviewed-on: http://gerrit.dmdirc.com/1799
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Local Commits <dmdirc@googlemail.com>