Add a QML debug config option QML_LOOKUP_HINTS to find non-final properties

This helps to optimize property access, especially for attached properties.

Change-Id: Id47a9c5f184f84ce5ab813d3b01d1a6c6031233e
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Slightly accelerate access to value type properties

We can't do a fast property index based access on them, due to the inability to
read individual fields from the original object (i.e.  the logic in
QQmlValueTypeWrapper). However what we can determine and propagate is the type
information of the individual properties, i.e. that the x and y properties of a
QPointF are always doubles.

Change-Id: Iee71ece2117294b7bc0b93deb0a77d7c51148b11
Reviewed-by: Lars Knoll <lars.knoll@digia.com>