Add soft boundaries to bauhaus sliders

These have to be enabled using dt_bauhaus_slider_enable_soft_boundaries
and you should use dt_bauhaus_slider_set_soft instead of
dt_bauhaus_slider_set for those afterwards. Growing the region is only
done when entering the value manually (right click or vim keys).
Use soft boundaries of [-3 , 3] for exposure

Hard boundaries are still at [-18 , 18 ].