Fix #877

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Switched from relying on winit's WindowEvent to DeviceEvent which happens immediately and doesn't do any key repeat. This required keeping track on which keys are pressed and which not, in order to automatically release all keys when the window loses focus. Synthetic WindowEvent::KeyboardInput events are still handled, so keys pressed when the window gains focus are not ignored.

A byproduct of this change is that key repeats will not happen anymore to GameInput stuff. I suspect it wasn't necessary to begin with, but if it is some adaptation will be required.

Key repetitions in widgets still works perfectly well.