Merge: gamnit keys: simple service to keep track of pressed keys

This simple service can be incredibly useful in games where a key held down causes an event at each frame, like walking forward by pressing `w`.

I use this in the 3D version of Tinks! Which should be ready for a PR in a near future.

Pull-Request: #1922
Reviewed-by: Jean Privat <jean@pryen.org>