Instead of everyone overriding save_to() and set_property() and doing
a pretty asymmetric job of implementing the various properties, let's
add a bit of structure here.
Object properties are now represented by a Core::Property. Properties
are registered with a getter and setter (optional) in constructors.
I've added some convenience macros for creating and registering
properties, but this does still feel a bit bulky. We'll have to
iterate on this and see where it goes.
We also clean up some old references to the old G prefixed GUI classes
This also fixes a potential bug with using: C_OBJECT_ABSTRACT(GAbstractButton)
instead of C_OBJECT_ABSTRACT(AbstractButton)
