Add Fast Zoom In & Out functions.

Shift +/-  or  Shift Mouse Wheel

http://pulkomandy.tk/projects/GrafX2/ticket/28

src/const.h

@@ -540,6 +540,8 @@ enum SPECIAL_ACTIONS
   SPECIAL_FORMAT_CHECKER_MENU,
 
   SPECIAL_HOLD_PAN,
+  SPECIAL_ZOOM_IN_MORE,
+  SPECIAL_ZOOM_OUT_MORE,
   
   NB_SPECIAL_SHORTCUTS            ///< Number of special shortcuts
 };