- Fix the Histogram background and grid lines
- Add 1 pixel of empty space between in the Histogram panel
- Add a light grey color border for Gtk::DrawingArea oject, for those
using the render_frame method
- In the Toolbox, make the nested MyExpander and Gtk::Frame lighter, and
adjust the Gtk::DrawingArea background color accordingly
- All added class to the MyCurve has been removed, so that the
Gtk::Drawing object's style is used
- The Retinex Settings' expander is now level 2 for GUI consistency
