ges-validate: add an add-asset action
scenarios: add a remove-asset action
validate: Add add-layer and remove-layer
ges-launch: Only create a layer if needed.

That way scenarios can start with an empty timeline
timeline-element: return TRUE in _set_name when both names match.
validate: implement remove / add clip actions

And a helper to get a layer by priority
ges-launch: Make it so actions are executed directly when needed.
scenarios: Check priority before creating a layer.
validate: make sure we release our ref when we get_timeline.