Do not process mouse down events if the slider is disabled

git-svn-id: svn+ssh://svn.gna.org/svn/gnustep/libs/gui/trunk@12128 72102866-910b-0410-8b05-ffd578937521

Source/NSSlider.m

@@ -417,6 +417,8 @@ static Class cellClass;
 
 - (void) mouseDown: (NSEvent *)theEvent
 {
+  if ([_cell isEnabled])
+    {
       NSPoint location = [self convertPoint: [theEvent locationInWindow] 
 			       fromView: nil];
       NSRect rect;
@@ -442,6 +444,6 @@ static Class cellClass;
 	}
       
       [self trackKnob: theEvent knobRect: rect];
+    }
 }
-
 @end