support multiple pointer devices

[karroffel]

The current Input<MouseButton> interface is very nice but implies the existence of a single mouse input device.

In future maybe multiple mice or touch inputs want to be supported (touch having a "backwards compatible" interface with MouseButton maybe?), for that a mechanism to select/retrieve the input device ID would be needed.

[inodentry]

Now that bevy has an API for touch input (and since mice and touch are different things anyway), can we adjust this issue (change the title, etc) to be about supporting multiple mice / pointer devices, specifically?

[bushrat011899]

This feels like a surprisingly straight-forward problem to be the oldest open issue on Bevy.