Dynamic window abstraction

[eliaspekkala]

Bevy's window abstraction isn't very dynamic; it doesn't let you set window properties on demand like you can with Winit. This means you have to use the Winit API directly (though WinitWindows) to do things like mouse grabbing (see #614).

@cart said the following about this issue:

In the medium term, we need to revisit our Window abstraction and either:

1. Expose a way to easily / dynamically set window properties at runtime
2. Fully embrace Winit as our windowing abstraction (which implicitly means that we must extend Winit whenever we want to support new platforms, such as consoles). This is a pretty big commitment, so I'm pretty hesitant to do this. But it would certainly simplify the code / immediately expose a bunch of nice features.

[karroffel]

It seems like #644 is relevant here

[cart]

I just left some comments there about the various paths we can take.

[cart]

Closed by #644