BUG 375842
Add an inverse button to level filter widget such that positions
of the black and the white cursor can be swapped.
Add 'inverted' field to KisGradientSlider. In inverted mode,
gradients are drawn in the reverse order, from white to black.
Fix confusing representation of coordinates of cursor in
KisGradientSlider
Signed-off-by: Wuwei Lin <vincentl13x@gmail.com>