GSColorSliderCell: Move to libs-gui (#321)

This class is shared between the StandardPicker bundle and the WheelPicker bundles.  Exposing a class with the same name in two separate bundles results in undefined behavior.  Move this class to a shared module (libs-gui in this case) to resolve that.

Source/GSColorSliderCell.h

@@ -0,0 +1,44 @@
+/* GSColorSliderCell.h
+
+   Copyright (C) 2007 Free Software Foundation, Inc.
+
+   Author:  Fred Kiefer <FredKiefer@gmx.de>
+   Date: December 2007
+   Author: Alexander Malmberg <alexander@malmberg.org>
+   Date: May 2002
+
+   This file is part of GNUstep.
+   
+   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 GSColorSliderCell_h
+#define GSColorSliderCell_h
+
+#include <AppKit/NSSliderCell.h>
+
+APPKIT_EXPORT_CLASS
+@interface GSColorSliderCell : NSSliderCell
+{
+  int mode;
+  float values[3];
+}
+-(void) _setColorSliderCellMode: (int)m;
+-(void) _setColorSliderCellValues: (float)a : (float)b : (float)c;
+@end
+
+#endif