Desktop GUI needs double-/range-sliders

[GPMueller]

For z-range sliders, double sliders would be nicer than two coupled sliders.

Here is an example implementation: http://stackoverflow.com/a/26025796

[IngoMeyer441]

I've tested the implementation provided on stack overflow and got this result:

In my opinion, this range slider is not optimal. The blue interval should be drawn between both handles and the second handle should be drawn with the Qt api directly and not using a png file.

The problem is that this approach tries to customize a normal QSlider. I think, it is necessary to create a custom widget to provide a good solution.

[GPMueller]

Implemented range sliders for ui-imgui on the feature-imgui branch with a62dd7d:

Note, range sliders are not yet implemented in ImGui itself, but for my implementation I took inspiration from comments on ocornut/imgui#76.