Rename ambiguously named overridden property variable and type

* glib/glibmm/class.[cc|h]:
* glib/glibmm/interface.cc:
* glib/glibmm/property.cc: Rename properties_quark and properties_type to
iface_properties_quark and iface_properties_type.
As custom_[set|get]_property_callback() in property.cc also sets and gets
user-defined properties, this naming choice may be confusing.
These names are intended for use by glibmm only. Renaming is not an
ABI/API break. Bug #731484.
PropertyBase: Use g_object_notify_by_pspec() instead of g_object_notify()

This approach is faster and recommended by GLib. Bug #731484.