Slider Controller API Change

Prepare to allow BarController (with Spinbutton) to translate
internal/interface values for the SpinBox.

(The change is irrelevant for the fader/slider itself which
always use the [user-]interface value)
implement spin-button internal<>interface bridge

Widgets were written to use "[User] Interface" values, but the
spinbutton in a Bar-controller should allow to directly access
the "Internal Value".
NOOP, just whitespace/indenting