Draft: Use PropertyProxy from Gaudi

Experimental MR to migrate to Gaudi's PropertyProxy (and ConfigurableDb). In principle there are only very few differences and this would avoid having to maintain our own copy each time a new property type gets added.

Ideally, we would do the same for the (legacy) Configurable class but that requires more cleanup first (i.e. removing the support of the CA behavior once we no longer need it).