Support gamepad input

[fenomas]

The engine should (by default) know how to accept inputs from gamepads. Steps for this:

• [ ] game-inputs needs to learn how to catch gamepad buttons/axis inputs, and emit events similarly to key/mouse events
• [ ] lib/inputs should add default bindings for gamepad keys/axes
• [ ] comps/receivesInputs comp should be able to set moveState.running to a fractional value if left-stick axis inputs are present
• [ ] comps/movement should change so that running is a 0..1 float instead of a boolean (and probably rename it)
• [ ] comps/movement should set the push vector based on the size of the running prop
• [ ] Add bindings to the demo apps for the feature demo inputs (currently bound to 1/2/3 keys)

Analog button press inputs should probably left out for now, as it would require lower-level changes..