WelcomePage: Add QMAKE variable for QQuickWidget support

To use QQuickWidget in the welcomepage just run
qmake "USE_QUICK_WIDGET=true".

I also removed the commented code for the event filter.

Change-Id: Ic56527704c0664b3eff4fdf6bef17e250b72db46
Reviewed-by: Eike Ziller <eike.ziller@digia.com>