NSScroller: define scroller styles

As of macOS 10.7, scrollers gained two types: Legacy and Overlay. Here,
we define these styles as well as a method to determine which is the
"default" style to be used.

Source/NSScroller.m

@@ -49,6 +49,9 @@
 
 #import "GNUstepGUI/GSTheme.h"
 
+NSString *NSPreferredScrollerStyleDidChangeNotification =
+  @"NSPreferredScrollerStyleDidChangeNotification";
+
 /**<p>TODO Description</p>
  */
 @implementation NSScroller
@@ -130,6 +133,12 @@ static float	buttonsOffset = 1.0; // buttonsWidth = sw - 2*buttonsOffset
   return scrollerWidth;
 }
 
++ (NSScrollerStyle)preferredScrollerStyle
+{
+  // FIXME: a theme should define this?
+  return NSScrollerStyleLegacy;
+}
+
 - (BOOL) isFlipped
 {
   return YES;