Use WindowProviders/factories for all windows.

Remove the legacy code, which was used in far fewer places than I thought.

Change-Id: I72cfa09e92110163e16a8a43f18a4127560a65c1
Reviewed-on: http://gerrit.dmdirc.com/3063
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>