[NumberInput] Hold stepper buttons to continuously change value

[mj12albert]

Closes #74

TODO

• make sure the tests are correct
• longer delay between the first onChange on the first mouseDown and subsequent continuously-firing ones
• pointer instead of mouse?
• handle mouse/pointerOut

[mj12albert]

clock.tick(300) doesn't work because if I don't run the first timer the subsequent ones will never be created

2nd try: I thought this was needed to work:

clock.tick(100)
clock.runToLast();
clock.tick(100)
clock.runToLast();
clock.tick(100)
clock.runToLast();

because I thought if I didn't run it after each 100ms, the timer would get cleared?