ui.framebuffersize is required.

If we pass required = false then the config manager will happily
return null if it encounters a disabled optional setting. We
don't accept null so that's a bit daft.

Change-Id: I37934ba15877b8c0caf188b5f80e3353c8555134
Fixes-Issue: CLIENT-491
Reviewed-on: http://gerrit.dmdirc.com/3333
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager