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/mydiagonalcurve.cc

@@ -235,6 +235,8 @@ void MyDiagonalCurve::draw (int handle)
     cr->paint ();
     cr->set_operator (Cairo::OPERATOR_OVER);
 
+    style->render_background(cr, graphX, graphY-graphH, graphW, graphH);
+
     Gdk::RGBA c;
 
     // histogram in the background
@@ -426,8 +428,8 @@ void MyDiagonalCurve::draw (int handle)
         // first the background
         int bWidth = CBAR_WIDTH;
         BackBuffer *bb = this;
-        leftBar->setDrawRectangle(Cairo::FORMAT_ARGB32, 1, graphY - graphH + 1, bWidth - 2, graphH - 2);
-        leftBar->expose(bb);
+        leftBar->setDrawRectangle(1, graphY - graphH + 1, bWidth - 2, graphH - 2);
+        leftBar->expose(*this, bb);
 
         // now the border
         c = style->get_border_color(state);
@@ -441,8 +443,8 @@ void MyDiagonalCurve::draw (int handle)
         // first the background
         int bWidth = CBAR_WIDTH;
         BackBuffer *bb = this;
-        bottomBar->setDrawRectangle(Cairo::FORMAT_ARGB32, graphX + 1, graphY + CBAR_MARGIN + 1, graphW - 2, bWidth - 2);
-        bottomBar->expose(bb);
+        bottomBar->setDrawRectangle(graphX + 1, graphY + CBAR_MARGIN + 1, graphW - 2, bWidth - 2);
+        bottomBar->expose(*this, bb);
 
         // now the border
         c = style->get_border_color (state);
@@ -511,8 +513,6 @@ bool MyDiagonalCurve::on_draw(const ::Cairo::RefPtr< Cairo::Context> &cr)
         }
     }
 
-    Glib::RefPtr<Gtk::StyleContext> style = get_style_context();
-    style->render_background(cr, 0., 0., (double)get_width(), (double)get_height());
     draw (lit_point);
     copySurface(cr);
     return false;