Dynamically register gamepads

[Piturnah]

Prior to migrating to 0.15 (#20), we would assign players 3 and 4 to gamepads automatically based on their indexes.

Since 0.15, bevy introduced "gamepads as entities", which improves the API of working with them.

A consequence of this is that gamepads can't just be referenced by a static index anymore (afaik).

So, in #20 we disabled the gamepads entirely as a temporary solution, but it would be good to get them working again.

Since we are still prototyping and don't have a "character select screen (#7)", an appropriate solution is to keep players 1 and 2 on the default keyboard maps, and if a gamepad is plugged in assign it dynamically to player 3, then a subsequent one to player 4. Don't particularly mind what happens with further gamepads after that, for the prototype.

It may be useful to refer to the leafwing input manager example.