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/curveeditorgroup.h

@@ -39,7 +39,7 @@ class FlatCurveEditorSubGroup;
  * - to start a new line of curve button, use the 'newLine' method
  * - if you add more than one curve, you must add a "CurveEditor* ce" parameter to your listener
  */
-class CurveEditorGroup : public Gtk::VBox, public CurveListener
+class CurveEditorGroup : public Gtk::Grid, public CurveListener
 {
 
     friend class CurveEditor;
@@ -49,6 +49,7 @@ class CurveEditorGroup : public Gtk::VBox, public CurveListener
 
 private:
     Glib::ustring& curveDir;
+    int line;
 
 protected:
     Gtk::Label* curveGroupLabel;
@@ -86,6 +87,7 @@ public:
     }
     //void on_realize ();
     CurveEditor* addCurve(CurveType cType, Glib::ustring curveLabel, Gtk::Widget *relatedWidget = nullptr, bool periodic = true);
+    void attachCurve (Gtk::Grid* curve);
 
 protected:
     //void curveTypeToggled ();