Move window management into seperate class.

This also changes the behaviour of the look and feel and
textpane font and size preferences and they now require a
restart.

Change-Id: I9fee668306493d49d36f4bfc29a8a819bc44eaae
Reviewed-on: http://gerrit.dmdirc.com/3308
Reviewed-by: Chris Smith <chris@dmdirc.com>
Automatic-Compile: DMDirc Build Manager