Represents an event function that is called when the application becomes active.
If null is specified, the function will not be called.
Unlike regular event handlers, if you want to receive this event, set this property to the function you want to call.
Window.onActivate is fired when each window in the same application is activated, but this event is fired when the main window is activated.
Note that this event can also occur if the main window is already active (there is no guarantee that it will occur completely onActivate → onDeactivate → onActivate → ……).