RangeSlider: Allow setting an independant min/max for the lower/upper slider

[jneufferrpm]

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This adds the following properties to the range slider:

• SlideableUpperMin
• SlideableLowerMax

This allows setting the min/max that the sliders can move to independently for the upper and lower slider. For example, you can set SlideableUpperMin to prevent the upper slider from going below a certain value.

Additionally, this fixes the behavior of the existing SlideableMin and SlideableMax, to prevent an issue where setting a min/max such that both the upper and lower slider would be outside of it would break the slider.