[Feature request] ComboBox on Open / Close callback

[schwarda]

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.

[rohitbhoite]

Fixed as a part of this pr
#1258