NSUserInterfaceItemIdentification: introduce protocol

Introduce the NSUserInterfaceItemIdentification protocol, which required
an user interface class provide an unique identifier.

Headers/AppKit/AppKit.h

@@ -192,6 +192,7 @@
 #import <AppKit/NSTreeController.h>
 #import <AppKit/NSTreeNode.h>
 #import <AppKit/NSUserDefaultsController.h>
+#import <AppKit/NSUserInterfaceItemIdentification.h>
 #import <AppKit/NSUserInterfaceValidation.h>
 #import <AppKit/NSViewController.h>
 #import <AppKit/NSWindowController.h>

Headers/AppKit/NSUserInterfaceItemIdentification.h

@@ -0,0 +1,44 @@
+/** NSUserInterfaceItemIdentification.h
+   <abstract>Associate a unique identifier with objects in your user interface</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_NSUserInterfaceItemIdentification
+#define _GNUstep_H_NSUserInterfaceItemIdentification
+
+@class NSString;
+
+#if OS_API_VERSION(MAC_OS_X_VERSION_10_7, GS_API_LATEST)
+@protocol NSUserInterfaceItemIdentification
+#if GS_HAS_DECLARED_PROPERTIES
+@property (copy) NSString *identifier;
+#else
+- (NSString *) identifier;
+- (void) setIdentifier: (NSString *)identifier;
+#endif
+@end
+#endif
+
+#endif

Source/GNUmakefile

@@ -430,6 +430,7 @@ NSNibDeclarations.h \
 NSNibLoading.h \
 NSNib.h \
 NSSpellProtocol.h \
+NSUserInterfaceItemIdentification.h \
 NSUserInterfaceValidation.h \
 DPSOperators.h \
 PSOperators.h