[WIP] New RangeSlider

Replaces the old RangeSlider with one based on code from [here](http://timokorinth.de/creating-range-slider-wpf-silverlight/). Thanks to Timo Korinth for his initial code, we may be able to create a similar RangeSlider that we may be able to fix in the future.

**Probably won't make it into the next stable release.**..*probably*.

### Todo list
- [ ] Introduce ```MinRange``` and ```MaxRange``` properties. These specify the shortest and great distance the two thumbs can be from each other.
- [ ] Add *Disabled* visual state.
- [ ] Other things...

![rs](https://f.cloud.github.com/assets/251501/1843006/a08f6f26-74c7-11e3-98d4-7648ef01f12f.PNG)
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cc @QuantumDeveloper