Bug 917463 - [Clock] Implement scrolling inertia in inline time picker for Timers - r=jugglinmike

https://bugzilla.mozilla.org/show_bug.cgi?id=917463

- Removes "slow down" effect when under threshold, is awkward
- Adds "inertia" by detecting the speed of the last pan event when a swipe finishes and adjusts the index properly
- Separated out the "stopInteraction" method from the onswipe