Check return value of g_dbus_connection_register_object(), use GCancellable

Submitted by David King @davidk

Link to original bug (#704785)

Description

As noticed by Bastien in bug 692771, comment 20, some uses of g_dbus_connection_register_object() do not check the return value, where 0 indicates an error. The orientation plugin is one such example:

plugins/orientation/gsd-orientation-manager.c:354

Just below that call, there is a g_dbus_proxy_new() which should use a GCancellable, as should the g_bus_get() call on line 447.