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/flatcurveeditorsubgroup.cc

@@ -337,7 +337,7 @@ void FlatCurveEditorSubGroup::switchGUI()
             CPointsCurve->setColoredBar(leftBar, bottomBar);
             CPointsCurve->queue_resize_no_redraw();
             updateEditButton(dCurve, editCPoints, editCPointsConn);
-            parent->pack_start (*CPointsCurveGrid);
+            parent->attachCurve (CPointsCurveGrid);
             CPointsCurveGrid->check_resize();
             break;