Sets the event mask 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 GTK2.NO_WINDOW widgets; to get events on those
widgets, place them inside a GTK2.EventBox and receive events on the event
box.