Improve the usability of the number scubber

I made the following changes:
- added clickable regions so that users can click on the left/right arrows to advance by one (or one decimal place)
- added modifier keys to allow the scrubber to move at different rates: +alt = 0.1, +shift = 10, +alt+shift = 1

Can you add a comment describing what this function does? I might also rename e to event or evt, to make it even more obvious it's an event object.