Sets the event mask (see EventMask) for a widget.
The event mask determines which events a widget will receive. Keep in mind that different widgets have different default event masks, and by changing the event mask you may disrupt a widget’s functionality, so be careful. This function must be called while a widget is unrealized. Consider add_events for widgets that are already realized, or if you want to preserve the existing event mask. This function can’t be used with widgets that have no window. (See get_has_window). To get events on those widgets, place them inside a EventBox and receive events on the event box.