Adds a timeline element to the container.
The element will now be a child of the container (and the container will be the parent of the added element), which means that it is now controlled by the container. This may change the properties of the child or the container, depending on the subclass.
Additionally, the children properties of the newly added element will be shared with the container, meaning they can also be read and set using get_child_property and set_child_property on the container.
| this | |
| child |
The element to add as a child |
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true if |