Fixes #1811 : Now a locked value won't be propagated to a descendant visual property that has a different type.

Change-Id: Idfbaeb129e4610b86350a3efe9cffea0647690ca
Refs #1758: XGMML Exporter now catches and logs ClassCastExceptions when serializing visual properties, so the export task doesn't fail just because of an incorrect visual property value.

Change-Id: I59654214e78de01a1ad27a58849f20404fecc623