implement integerValue method in NSSliderCell (#296)

the superclass method integerValue: method in NSCell uses the _object_value to retrieve the result, but the implementation of setObjectValue: in NSSliderCell sets _value. So integerValue: needs to return _value.

Source/NSSliderCell.m

@@ -658,6 +658,11 @@ double _doubleValueForMousePoint (NSPoint point, NSRect knobRect,
     }
 }
 
+- (NSInteger) integerValue
+{
+  return (NSInteger)_value;
+}
+
 - (id) objectValue
 {
   return [NSNumber numberWithDouble: _value];