Mouse capture API

[eliaspekkala]

The pull request "Add mouse capture example" and the recipe "Grabbing the mouse" in the Bevy cookbook both illustrate how mouse capturing is done currently. It's done using the Winit API directly (though WinitWindows), which ties these Bevy apps to using Winit's backends.

Bevy should have a mouse capture API that lets us do the same thing, but on a higher level of abstraction that isn't tied directly to Winit.

In the short term, @cart is cool with adding more properties to the existing WindowDescriptor to unblock scenarios like mouse grabbing, but in the medium term one issue is that Bevy doesn't let you set window properties on demand like you can with Winit (see comment and issue #641).

[cart]

Closed by #679