purge DBusObjectID

	* dbus/dbus-connection.c: port to no ObjectID, create a
	DBusObjectTree, rename ObjectTree to ObjectPath in public API

	* dbus/dbus-connection.h (struct DBusObjectTreeVTable): delete
	everything except UnregisterFunction and MessageFunction

	* dbus/dbus-marshal.c: port away from DBusObjectID,
	add DBUS_TYPE_OBJECT_PATH

	* dbus/dbus-object-registry.[hc], dbus/dbus-object.[hc],
	dbus/dbus-objectid.[hc]: remove these, we are moving to
	path-based object IDs