QQuickApplication: do not filter ApplicationActive/Inactive events

Listening to QGuiApplication::applicationStateChanged() is enough, no
need to do expensive event filtering for listening to application state
changes.

Change-Id: Iba71bc8cefec236f2f7e2e25c57c5c6b9c9fb1ea
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>