Proof-of-concept multi-thumb slider

[jdelight]

What I did

• implemented a very simple proof of concept multi-thumb slider
• uses two input ranges layered on top of each other with some dynamic styles to indicate selected range
• uses LionFieldset to present multiple inputs as a single control
• requires a custom LionInputRange component to remove value/unit text or an attribute
  on the LionInputRange class to hide it
• uses a similar technique to the one suggested here: https://css-tricks.com/multi-thumb-sliders-particular-two-thumb-case/

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[gerjanvangeest]

This PR still needs some attention, maybe we can bring it back to live and finalise it soon!
Anybody lending a hand, feel free 💪!