Make egui-winit more flexible regarding custom WindowEvent

[lmaxyz]

Any chance you'll approve pull request about adding this code at the end of on_window_event method?

_ => {
    // ToDo: Add warning logging when receive unknown WindowEvents.
    EventResponse {
        // We probably want to repaint by any unknown event.
        repaint: true,
        consumed: false,
    }
}

It make me easy to add custom WindowEvent values in my winit's fork.
But maybe it creates possibility to miss new winit events and miss it's handlers adding..