Remove the remaining WindowManager singleton refs.

Add a GlobalWindowManager to keep state and references, rather than
use static methods on GlobalWindow.

Minor tidying.

Change-Id: I65b84082f0a07cdc9e4c05263f6c5c77282e9b04
Reviewed-on: http://gerrit.dmdirc.com/2818
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>