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/coloredbar.h

@@ -25,12 +25,17 @@
 /*
  * Parent class for all colored bar type; a ColorProvider has to be set
  * thanks to "setColorProvider" to be able to display colors inside the bar
+ *
+ * WARNING: If the color has no gradient defined or can't get colors from the provider,
+ *          the bar will have undefined data, and the calling class will have to draw
+ *          the bar itself, i.e. use render_background (depending on its Gtk::Style)
+ *
  */
-class ColoredBar : public BackBuffer, public ColorCaller
+class ColoredBar : private BackBuffer, public ColorCaller
 {
 
 private:
-    void draw();
+    void updateBackBuffer(Gtk::DrawingArea &drawingArea);
 
 protected:
     eRTOrientation orientation;
@@ -38,11 +43,12 @@ protected:
 
 public:
     explicit ColoredBar (eRTOrientation orient);
+    bool setDrawRectangle(int newX, int newY, int newW, int newH, bool updateBackBufferSize = true);
 
-    void expose(Glib::RefPtr<Gdk::Window> destWindow);
-    void expose(Cairo::RefPtr<Cairo::ImageSurface> destSurface);
-    void expose(BackBuffer *backBuffer);
-    void expose(const Cairo::RefPtr< Cairo::Context> &cr);
+    void expose(Gtk::DrawingArea &drawingArea, Glib::RefPtr<Gdk::Window> destWindow);
+    void expose(Gtk::DrawingArea &drawingArea, Cairo::RefPtr<Cairo::ImageSurface> destSurface);
+    void expose(Gtk::DrawingArea &drawingArea, BackBuffer *backBuffer);
+    void expose(Gtk::DrawingArea &drawingArea, const Cairo::RefPtr< Cairo::Context> &cr);
 
     bool canGetColors();
 
@@ -51,6 +57,10 @@ public:
     // by clearing the gradient, the ColorProvider will have to provide colors on a per pixel basis if a ColorProvider
     // has been set, through ColorProvider::colorForValue on next ColoredBar::expose
     void clearBgGradient ();
+
+    void setDirty(bool isDirty) {
+        BackBuffer::setDirty(isDirty);
+    }
 };
 
 #endif