Sets the function to call to handle all events from GDK.
Note that GTK+ uses this to install its own event handler, so it is usually not useful for GTK+ applications. (Although an application can call
this function then call
gtk_main_do_event to pass events to GTK+.)
the function to call to handle events from GDK.
user data to pass to the function.
the function to call when the handler function is removed, i.e. when handler_set is called with another event handler.