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/rtwindow.cc

@@ -125,7 +125,11 @@ RTWindow::RTWindow ()
     }
 
     set_title_decorated("");
+    set_resizable(true);
+    set_resize_mode(Gtk::ResizeMode::RESIZE_QUEUE);
+    set_decorated(true);
     set_default_size(options.windowWidth, options.windowHeight);
+    set_hide_titlebar_when_maximized(true);
     set_modal(false);
 
     if (options.windowMaximized) {