GUI bugfix (histogram) + touchup

- Fix the Histogram background and grid lines
- Add 1 pixel of empty space between in the Histogram panel
- Add a light grey color border for Gtk::DrawingArea oject, for those
using the render_frame method
- In the Toolbox, make the nested MyExpander and Gtk::Frame lighter, and
adjust the Gtk::DrawingArea background color accordingly
- All added class to the MyCurve has been removed, so that the
Gtk::Drawing object's style is used
- The Retinex Settings' expander is now level 2 for GUI consistency

rtgui/mydiagonalcurve.cc

@@ -37,11 +37,6 @@ MyDiagonalCurve::MyDiagonalCurve () : activeParam(-1), bghistvalid(false)
     editedPos.resize(2);
     editedPos.at(0) = editedPos.at(1) = 0.0;
 
-    Glib::RefPtr<Gtk::StyleContext> style = get_style_context();
-    style->add_class(GTK_STYLE_CLASS_FRAME);
-    //style->add_class(GTK_STYLE_CLASS_NOTEBOOK);
-    //style->add_class(GTK_STYLE_CLASS_FLAT);
-
     signal_event().connect( sigc::mem_fun(*this, &MyDiagonalCurve::handleEvents) );
 
     curve.type = DCT_Spline;