Add a function to know if any popup is open.

[francesco-cattoglio]

Is your feature request related to a problem? Please describe.
In my software, I need to know if any combo box is currently open. I would like to have a function similar to Memory::is_popup_open(), which however returns true if any popup is open.

Describe the solution you'd like
I am making a pull request to add this simple function. Could you accept it? 😛

Describe alternatives you've considered
As an alternative, I can have boolean variables floating around in my code alongside my combo boxes, but I find it somewhat ugly.