[Feature request] ComboBox on Open / Close callback

It would be great to know if ComboBox just opened or closed.
In SplitButtons, I could get information about whether it is open or not via flyoutController.isOpen through globalKey (also complicated), but ComboBox has no way to get this information.
For example DropDownButtons are the best because I can easily access to arguments onOpen, onClose.

Start by locating the ComboBox implementation and compare it with DropDownButton's existing onOpen and onClose arguments. The work is done when ComboBox exposes callbacks that let callers determine when it opens and closes, with coverage for both transitions.

Written by the indexing model from the issue text.