Slider: Fix non-live sliders, rework bridged values

Similar to the latest ProgressBar rewrite:

> Get rid of nasty math! Embrace simplicity and perfection of `position`!
>
> Now the range of possible bridged values is totally predictable, safe
> from overflows, and precise enough to express fractions of progress
> even on the biggest available monitors.

Expect, here we still have to deal with to/from/stepSzie, because
bridged step needs to be scaled.

But most importantly, `position` receives real-time updates even for
non-live Sliders, while `value` gets stuck until a Slider is released.

BUG: 473400