Get rid of special handling of var properties

Get rid of special handling of var properties

These can be handled in a simple way now by using a special
propertyType value indicating that we have a var property.

Also remove the additional write calls in the different
readProperty implementations. If the stored data doesn't
match, we can simply return the default value directly.

Change-Id: I3823a971df24bd78f0acdc4c0042776277b3c55f
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>