Add event callback function properties to Property Pane control options interfaces
The IPropertyPaneButtonProps interface exposes an onClick property which can be set to the name of a callback function that can be executed when a user clicks on the button in the property pane.
I am requesting that you implement callbacks for the other kinds of controls in the property pane so that we can customize the response to changes in those controls. Examples:
CheckBox, Toggle: onStateChange(state)
Dropdown List, Choice Group: onSelectedItemChange(selectedItem)
and so on...