New batch of update for issue 3446

This commit include :
- cleanup rtgui/retinex.cc file (constructor only) and switched to
Gtk::Grid (the new standard) instead of Gtk::Box. This however doesn't
solve the issue of the Transmission curves in the Retinex tool, with
wrong resize of the Frame when folding/unfolding the curves.
- better alignment of the Histogram panel now with 4px of padding on the
left and right side
- Threshold selector now use the Scale's Trough style to draw its box
and sliders
- Curve's background are darker and restricted to the curve diagram
- Diagonal and Flat curves has been converted to Gtk::Grid as well
- A special color is now used for unsensitive Threshold selector and
Sclaes widgets
- Gap around the main Window has been removed on windows to circumvent a
bug in Gtk3, but the window can now only be maximized (usual use case)
or resized vertically by the top border only.
- Buttons at the bottom of the Editor panel has now the same height

rtgui/thresholdadjuster.h

@@ -140,11 +140,11 @@ public:
     }
     void setBgGradient (const std::vector<GradientMilestone> &milestones)
     {
-        tSelector.setBgGradient (milestones);
+        tSelector.coloredBar.setBgGradient (milestones);
     }
     void setBgColorProvider (ColorProvider *cp, int i)
     {
-        tSelector.setColorProvider(cp, i);
+        tSelector.coloredBar.setColorProvider(cp, i);
     }
     void setUpdatePolicy (eUpdatePolicy policy)
     {