Handle different type of sliders

[tbaranes]

There one thing which is really annoying with UISlider: it's hardly customisable.
So here a proposal to improve AnimatableSlider: add a property SliderType which is a bit similar to MaskDesignable but fully dedicated to sliders. Here some random examples I found on github:

• https://github.com/alskipp/ASValueTrackingSlider
• https://github.com/eliotfowler/EFCircularSlider
• https://github.com/spromicky/StepSlider
• https://github.com/TomThorpe/TTRangeSlider
• https://github.com/jmenter/JAMAccurateSlider
• ... there a lot of more possibilities!

Moreover, it's joining NSSlider concept which has a NSSliderType property to create linear or circular slider.

That would definitely be useful, and even more useful since we could design them directly from IB!

[JakeLin]

I feel it is annoying too. I like the idea to have more customizable Slider.

[lastMove]

Great Idea. I will maybe look at this.

[phimage]

one PR about it
#417