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/previewwindow.h

@@ -56,6 +56,11 @@ public:
     bool on_motion_notify_event (GdkEventMotion* event);
     bool on_button_press_event  (GdkEventButton* event);
     bool on_button_release_event(GdkEventButton* event);
+    Gtk::SizeRequestMode get_request_mode_vfunc () const;
+    void get_preferred_height_vfunc (int& minimum_height, int& natural_height) const;
+    void get_preferred_width_vfunc (int &minimum_width, int &natural_width) const;
+    void get_preferred_height_for_width_vfunc (int width, int &minimum_height, int &natural_height) const;
+    void get_preferred_width_for_height_vfunc (int height, int &minimum_width, int &natural_width) const;
 
     // PreviewListener interface
     void previewImageChanged ();