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/profilestore.cc

@@ -516,6 +516,8 @@ ProfileStoreLabel::ProfileStoreLabel(const ProfileStoreEntry *entry) : Gtk::Labe
 ProfileStoreComboBox::ProfileStoreComboBox ()
 {
     updateProfileList();
+    Gtk::CellRendererText* cellRenderer = dynamic_cast<Gtk::CellRendererText*>(get_first_cell());
+    cellRenderer->property_ellipsize() = Pango::ELLIPSIZE_MIDDLE;
 }
 
 Glib::ustring ProfileStoreComboBox::getCurrentLabel()