This patch fixes a value glitch when changing the slider value via
dragging the knob with the mouse and having a min value smaller than 0.
Before this patch it was not possible to drag the value below 0 and it
just snapped to the configured min value if the mouse was at the most
left position. Now the value is calculated from the value range and
mouse position within the widget.
This is a helpful option to prevent unwanted side effects, distinguish
between user and programmatic input, etc. Sliders and SpinBoxes were
implementing it idiosyncratically, so let's generalize the API and
give Buttons and TextEditors the same ability.
ValueSlider is a more generalized version of OpacitySlider when we need
a slider with values displayed. It will always show the current value
with a user defined suffix.