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.cc

@@ -290,3 +290,30 @@ bool PreviewWindow::on_button_release_event (GdkEventButton* event)
 
     return true;
 }
+
+Gtk::SizeRequestMode PreviewWindow::get_request_mode_vfunc () const
+{
+    return Gtk::SIZE_REQUEST_CONSTANT_SIZE;
+}
+
+void PreviewWindow::get_preferred_height_vfunc (int &minimum_height, int &natural_height) const
+{
+    minimum_height= 50;
+    natural_height = 100;
+}
+
+void PreviewWindow::get_preferred_width_vfunc (int &minimum_width, int &natural_width) const
+{
+    minimum_width = 80;
+    natural_width = 120;
+}
+
+void PreviewWindow::get_preferred_height_for_width_vfunc (int width, int &minimum_height, int &natural_height) const
+{
+    get_preferred_height_vfunc(minimum_height, natural_height);
+}
+
+void PreviewWindow::get_preferred_width_for_height_vfunc (int height, int &minimum_width, int &natural_width) const
+{
+    get_preferred_width_vfunc (minimum_width, natural_width);
+}