allow installing GProperty on interface types
Submitted by Emmanuele Bassi
we easily can make setter and getter functions be virtual functions, or wrappers to virtual functions.
the main problem is that the requirement to override the GParamSpec in the classes implementing the interface is still there — and it's actually encoded inside that wretched hive of debauchery that is GParamSpecPool. we will need to disentangle that particular knot first.