MAGN-5604 Remove execution throttle.

For the firefly work, and our work with Robots in Architecture, it has been requested that we remove the execution throttling we previously employed to limit the number of executions. We are stable enough now with the scheduler that I propose we remove this timer to restore the behavior that execution is called on every slider tick.

This PR does the following:

- Removes the execution throttle per [MAGN-5604](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5604).
- Refactors sliders to inherit from common base class and add the Step parameter in the slider settings per [MAGN-5617](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5617).
- Changes "Double Slider" to "Number Slider" per [MAGN-5601](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5601).

![image](https://cloud.githubusercontent.com/assets/1139788/5352996/7022a5d8-7f25-11e4-9fe6-3ae28bba53fe.png)

![image](https://cloud.githubusercontent.com/assets/1139788/5353006/8d3d3f70-7f25-11e4-962b-7792cf6648ba.png)


PTAL:

- [ ] @Benglin (To verify this isn't going to cause issues with the scheduler.)
- [x] @lukechurch (Because we put this throttle in earlier when working on optimistic concurrency.)

Cherry Pick:

- [ ] Master 
We lose the ability to undo a drag operation on a slider because `OnThumbDragStarted` is missing here.