add_child_property

Register a property of a child of the element to allow it to be written with set_child_property and read with get_child_property.

A change in the property will also appear in the deep_notify signal.

pspec should be unique from other children properties that have been registered on this.

Parameters:

this	A TimelineElement
pspec	The specification for the property to add
child	The Object who the property belongs to

Returns:

true if the property was successfully registered.