Instead of focusing on the instances of the clips and their children,
we relax the check to allow moving track element clip between clips
that share a common asset. This makes it as correct conceptually but
more flexible, and the code becomes simpler.
We used to always call the `->set_child_property` virtual method
of the object that `ges_timeline_element_set_child_property` was called
from, but that means that, in the case of referencing GESContainer
children properties from its children, the children wouldn't know
what child property have been set, and the children override wouldn't
be takent into account, in turns, it means that the behaviour could be
different in the setter depending on parent the method was called,
which is totally unexpected.
We now make sure that the vmethod from the element that introduced the
child property is called whatever parent method is called, making the
behaviour more uniform.
Fix the python override to make sure that new behaviour is respected.
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.
