Draft: Improves preview hi/lo slider behaviour

If a histogram or GHS stretch has been done, and if the result is a big (>5%) pedestal, and if the viewer is in linear mode, and if the lo slider is not set to zero, a notification dialog is shown and the slider values are set to 0 / rangemax.

When starting to compose an image in the star recomposition tool, if the viewer is in linear mode, the hi and lo sliders are set to 0 / rangemax.

No change has been made to the basic asinh stretch.

Complete, but draft pending user feedback regarding whether this addresses #985 (closed).