Custom Interfaces: Implement derived interface properties in present.

	* glib/glibmm/class.cc (Class::properties_quark): Initialize this
	GQuark which is used to store/get the data used for setting and
	getting the properties of the interfaces that a custom type overrides.
	(Class::interface_finalize_function): Add this function which
	once invoked frees the property data stored as gqata in the GType and
	allocated/appended to in the Glib::Interface constructor below.
	* glib/glibmm/class.h: Declare the interface_finalize_function above.
	Also declare the quark used to store/get the property data and the
	typedef data type of the property data.

	* glib/glibmm/interface.cc (Interface_Class::add_interface): Specify a
	custom interface finalize function when adding the interface so that
	the resources allocated for handling the derived interface properties
	can be freed if the type is a custom interface type.
	(Interface::Interface(const Interface_Class&)): Modify the constructor
	so that when dealing with a custom interface type, it gets a list of
	the properties of the interface to be added and overrides these by
	appending approperiate GValues to the data used to handle
	getting/setting properties that is stored as qdata in the GType.  The
	constructor uses g_param_spec_overrided() to override the properties
	of the implemented interface and g_object_install_property() to
	install the properties.

	* glib/glibmm/property.cc (PropertyBase::install_property): Rewrite
	this method so that the acquired generated id's of custom implemented
	properties does not collide with the id's of properties of implemented
	interfaces that have been overridden in a custom type.  This is done
	by offsetting the acquired generated id (by addition) with the number
	of already existing properties (the ones that have been overridden).
	(custom_get_property_callback): Rewrite this function (which gets
	properties for custom types) so that if the property id is less than
	or equal to the number of overridden interface properties (which would
	mean that an overridden interface property should be gotten) the
	correct overridden interface property is gotten.  Otherwise, a custom
	property should be retrieved, in which case the id is offset (by
	subtraction) when the PropertyBase is retrieved from the id which
	would ensure getting the correct PropertyBase.
	(custom_set_property_callback): Rewrite this function as the above
	custom_get_property_callback was rewritten.