Make custom interface properties instance data

* glib/glibmm/class.cc: Copy the default values of the interface properties
to the class's iface_properties_quark.
* glib/glibmm/property.cc: custom_set_property_callback(): Copy the
default values to object-specific data.
custom_get_property_callback(): Get object-specific value if it exists, else
class-specific default value. Bug #732746.