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/mycurve.h

@@ -68,7 +68,7 @@ protected:
     ColoredBar *leftBar;
     ColoredBar *bottomBar;
     CursorShape cursor_type;
-    int graphX, graphY, graphW, graphH; /// position and dimensions of the graphic area, excluding surrounding space for the points of for the colored bar
+    int graphX, graphY, graphW, graphH; /// position and dimensions of the graphic area, excluding surrounding space for the points or for the colored bar
     int prevGraphW, prevGraphH;         /// previous inner width and height of the editor
     Gdk::ModifierType mod_type;
     int cursorX;        /// X coordinate in the graph of the cursor