Navigator should use a different icon while panning

The Navigator uses the same hand icon regardless whether panning
the red square or not, this branch tries to fix that, #4738
Includes a cleanup of icon names, i.e. the crosshair should not be
called the hand.

rtgui/gradient.cc

@@ -346,7 +346,7 @@ CursorShape Gradient::getCursor(const int objectID)
         return CSMove2D;
 
     default:
-        return CSOpenHand;
+        return CSHandOpen;
     }
 }