NSVisualEffectView: create class stub

Create stub for the NSVisualEffectView class.

Source/NSVisualEffectView.m

@@ -0,0 +1,76 @@
+/*
+   NSVisualEffectView.m
+  
+   View with visual effects
+
+   Copyright (C) 2017 Free Software Foundation, Inc.
+
+   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.
+*/
+
+#import <AppKit/NSVisualEffectView.h>
+
+@implementation NSVisualEffectView
+- (NSVisualEffectMaterial) material
+{
+  return 0;
+}
+
+- (void) setMaterial: (NSVisualEffectMaterial)material
+{
+  return;
+}
+
+- (NSInteger)interiorBackgroundStyle
+{
+  return 0;
+}
+
+- (NSVisualEffectBlendingMode) blendingMode
+{
+  return 0;
+}
+- (void) setBlendingMode: (NSVisualEffectBlendingMode) mode
+{
+  return;
+}
+
+- (NSVisualEffectState) state
+{
+  return 0;
+}
+
+- (void) setState: (NSVisualEffectState)state
+{
+  return;
+}
+
+- (NSImage *) maskImage
+{
+  return nil;
+}
+
+- (void) setMaskImage: (NSImage *)image
+{
+  return;
+}
+@end