_gtkContainerAdd adds a widget object in a Containers
object. One child only can be added by this way. Generally, this function is used by
a toplevel container (a window by example) to add a sub container like a box, table or others.
Prototype : fun [* $] *
* : a widget Containers object (the parent).
$ : a widget (the child) to add in the parent. It can be a Containers itself.
Return * : the parent object or nil if an error occurs.
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