QskSlider: configurable origin

[uwerat]

It shall be possible to define an origin for a slider - similar to the origin of QskProgressBar. If defined the filling covers the interval [origin, value ] - otherwise the interval is [minimum, value].

M3 sliders have a specific configuration "Centered", that sound like a candidate for having the origin in the center of [minimum, mximum]. However there seems to be no filling then.

Anyway - the origin feature is easy to have and makes sense in many situations.