Now that the notion of layer has been moved down to #GESTimelineElement
(through the new #ges_timeline_element_get_layer_priority method), this
method make much more sense directly in the base class.
Implementing it in the overrides as PyGObject won't be able to properly
convert python values to GValues in some cases. Using
g_object_set_property works as some logic is implemented inside
PyGObject for that particular case. This is a "regression" due
to https://bugzilla.gnome.org/review?bug=769789&attachment=348766 were
we end up with an OverflowError while setting G_TYPE_UINT children
properties.
