Consistently use ve.ui.WindowManager everywhere

We were using OO.ui.WindowManager except where we absolutely
needed to use ve.ui.WindowManager. Instead, consistently
use VE's WindowManager class everywhere in VE, so future
customizations to ve.ui.WindowManager will be applied
everywhere in VE.

Change-Id: Id6660a4b54a496eefa6c79e162570d402f95eab5
Merge "Consistently use ve.ui.WindowManager everywhere"