NSUserInterfaceLayout: create interface layout constants

Define interface layout enums to be used by AppKit. This improves
compatibility with the reference platform.

Headers/AppKit/AppKit.h

@@ -193,6 +193,7 @@
 #import <AppKit/NSTreeNode.h>
 #import <AppKit/NSUserDefaultsController.h>
 #import <AppKit/NSUserInterfaceItemIdentification.h>
+#import <AppKit/NSUserInterfaceLayout.h>
 #import <AppKit/NSUserInterfaceValidation.h>
 #import <AppKit/NSViewController.h>
 #import <AppKit/NSWindowController.h>

Headers/AppKit/NSUserInterfaceLayout.h

@@ -0,0 +1,51 @@
+/** NSUserInterfaceLayout.h
+   
+   <abstract>AppKit user interface layout constants</abstract>
+
+   Copyright <copy>(C) 2017 Free Software Foundation, Inc.</copy>
+
+   Author: Daniel Ferreira <dtf@stanford.edu>
+   Date: 2017
+
+   This file is part of the GNUstep GUI Library.
+
+   This library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2 of the License, or (at your option) any later version.
+
+   This library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.	 See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with this library; see the file COPYING.LIB.
+   If not, see <http://www.gnu.org/licenses/> or write to the 
+   Free Software Foundation, 51 Franklin Street, Fifth Floor, 
+   Boston, MA 02110-1301, USA.
+*/
+#ifndef _GNUstep_H_NSUserInterfaceLayout
+#define _GNUstep_H_NSUserInterfaceLayout
+
+#import <Foundation/Foundation.h>
+
+#if OS_API_VERSION(MAC_OS_X_VERSION_10_6, GS_API_LATEST)
+typedef NSInteger NSUserInterfaceLayoutDirection;
+enum
+{
+  NSUserInterfaceLayoutDirectionLeftToRight = 0,
+  NSUserInterfaceLayoutDirectionRightToLeft = 1
+};
+#endif
+
+#if OS_API_VERSION(MAC_OS_X_VERSION_10_9, GS_API_LATEST)
+typedef NSInteger NSUserInterfaceLayoutOrientation;
+enum
+{
+  NSUserInterfaceLayoutOrientationHorizontal = 0,
+  NSUserInterfaceLayoutOrientationVertical = 1
+};
+#endif
+
+#endif

Source/GNUmakefile

@@ -416,6 +416,7 @@ NSTrackingArea.h \
 NSTreeController.h \
 NSTreeNode.h \
 NSUserDefaultsController.h \
+NSUserInterfaceLayout.h \
 NSView.h \
 NSViewController.h \
 NSWindow.h \