Change return type of Layer.addImage to void. Re #1037.

An ImageWidget is not a QObject, so our proxy method won't work.
Ensure that pointers to ImageWidgets created in C++ cannot 'get out'.
The addImage method can still be called with an ImageWidget constructed
in python, or a filename.

Code/Mantid/MantidPlot/src/qti.sip

@@ -725,8 +725,8 @@ public:
   void addHistogram(Matrix*);
   void addArrow(ArrowMarker*);
 
-  ImageMarker* addImage(ImageMarker*);
-  ImageMarker* addImage(const QString& fileName);
+  void addImage(ImageMarker*);
+  void addImage(const QString& fileName);
 
   void setTitle(const QString& t);
   void setTitleFont(const QFont &fnt);