mouse focus event must be sent all the time, not just when the user is subscribed to the event #8

The .mouse_focus event is critical to handling mouse interactions properly. That means it must be part of the event struct provided by the user.

Either make it so it must be there by removing the @hasField check, or figure out some way to send the event to all the widgets without going through the user's updateFn — if that's even possible.