gdbus-codegen: Ensure that generated skeletons are MT-safe

For example, if setting a property on a skeleton from another thread
than where it was constructed, the idle handler responsible for
emitting the PropertiesChanged() signal could run immediately and
clear skeleton->priv->changed_properties_idle_source causing
g_source_unref() to be called with a NULL pointer. This race was
easily be fixed by adding a lock to the skeleton object.

In addition to fixing this race, also move the code for setting up the
idle handler to a class handler for the GObject::notify signal. This
change allows use of g_object_freeze_notify() and g_object_thaw_notify()
to perform atomic property changes from another thread than the one
that the skeleton was created in.

Signed-off-by: David Zeuthen <davidz@redhat.com>