Revamp QQuickDwmFeatures

Add missing change notifiers, fix property names vs. isFoo()
accessor names, remove unused member variables, fix the event
filter and add missing updates...

Change-Id: Ie86a89dc1415ad082ac5c2e839245d55a446a1d4
Reviewed-by: Ivan Vizir <define-true-false@yandex.com>
Reviewed-by: Caroline Chao <caroline.chao@digia.com>