Slider control enhancements

[mpo]

Have been cleaning up some in the slider-control after the jquery ui update.

Some further future enhancements I've spotted.

• (for range) splitting the input-control in 2 separate boxes rather then one aggregated one (we then need to split again)
• allowing to hide the input-boxes completely or allowing them to be replaced by an echo- only (ie no user-input) <span/>

[mpo]

Just reminding myself that these topics are somewhat related to how we address the remainder of #204 (hence why I didn't tackle that yet)