"SnapTo" feature enabled in the curve editor + non periodic handling of the flat curve editor.

Here is the new modifiew key behaviour :

    * CONTROL while dragging a point :
      slow down 20x the point move (i.e. 1pixel of cursor move move the control point by 0.05px)

    * SHIFT while dragging a point :
      snap the point to the following elements, on a "use nearest solution" method :
          o Diagonal curve editor :
                + top bound,
                + bottom bound,
                + identity (diagonal) line,
                + same Y than previous point,
                + same Y than next point,
                + line made by the previous and next point ; this will help to create linear parts in the curve more easilly
          o Flat curve editor (when moving a point) :
                + top bound,
                + bottom bound,
                + identity (horizontal) line,
                + same Y than previous point (or last point if you move the first point),
                + same Y than next point (or first point if you move the last point
          o Flat curve editor (when moving a tangential handle) :
                + 0.0, 0.35, 0.5, 1.0 values
                  0.35 is the default handles value and does create an smooth "diagonal" transition between points.

rtgui/curveeditorgroup.h

@@ -70,7 +70,7 @@ public:
 	void setColorProvider (ColorProvider* p) { cp = p; }
 	CurveEditor* getDisplayedCurve () { return displayedCurve; }
 	//void on_realize ();
-	CurveEditor* addCurve(CurveType cType, Glib::ustring curveLabel);
+	CurveEditor* addCurve(CurveType cType, Glib::ustring curveLabel, bool periodic = true);
 
 protected:
 	//void curveTypeToggled ();