Introduce factories for frames.

Make the factories implement WindowProvider, and simplify the
SwingWindowFactory.

Change-Id: I0ee55bc971ab57e5c79c818693981797a0946c3f
Reviewed-on: http://gerrit.dmdirc.com/3704
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager