write some PointerHandler(s) which use QObject::installEventFilter()

I'm not sure if we will ever really need this, because we are trying to solve all the use cases via normal event delivery (let QQuickWindow be in charge of orchestrating it). But in practice, QtQuick Controls 2 and QtLocation have both made some use of event filtering; so it might turn out that some use cases can't be solved in another way, or that this way is actually the most elegant.