Slider: allow for easy modification of the default linear behaviour

Currently there is no way to easy override the linear behavior of the slider. There are times where another visual behavior (e.g. logarithmic) is more appropriate.

Consider the following example scenario: having a price filter, represented by a range slider, for used cars site. Most of the items will be relatively evenly distributed into a given range, but if you add something with large deviation (e.g. Ferrari f50 among bunch of old) it will extend the range so much that the resolution in the lower range will become useless - one step of the slider will filter-out 50+% of the items. This is a common enough scenario to be addressed properly in a sane way.

Currently there in no option nor an extension point for that. The calculation code is located inside _refreshValue() and _mouseCapture() and the modification requires to extend the plugin and override all the code in those methods. As the calculations are only tiny fraction of them this would make maintenance painful.

It'd be better if the distance calculation code is located in separate methods thus allowing extending the plugin without the need to copy/paste all the unrelated (to the distance calculation) code in _refreshValue() and _mouseCapture() just to modify the handle progress/distance behaviour.

Additionally options may be introduced to hold a calculation callbacks that will default to plugin's own linear calculation methods. That way modifying plugins behavior will only require setting new callbacks which is even easier.

Modification in that way will not cause BC break as no existing method will change it's behaviour/API