Add Window::get_fullscreen() -> bool

[rukai]

set_fullscreen takes an Option<WindowId>.
However we dont need to make get_fullscreen() return an Option<WindowId> because we can use get_current_monitor() -> WindowId instead.

The window manager (at least on X11) can set the fullscreen state so just remembering the state in winit will not be enough.

[francesca64]

The way I'd want to address this is similar to #208, since:

• Maximization events are necessary and it would make sense for the fullscreen API to follow the same pattern.
• There's also a fullscreen button on macOS.
• I tend to favor events over getters, since people presumably usually only want to know about changes, and only as often as they happen.

So I'd go with WindowEvent::Fullscreen(Option<MonitorId>). However, I'm not opposed to having the getter as well, though I think it really should return Option<MonitorId>, for consistency with set_fullscreen. That also makes it easier to do dumb stuff with combinators.

[rukai]

Thanks, that all makes sense.

[francesca64]

Are there any WMs you recommend for testing this?

[rukai]

Not sure if your asking me directly? I only use i3.