Add core services header in plugin header folder

Source/Plugins/Headers/CoreServicesHeader.h

+/*
+------------------------------------------------------------------
+
+This file is part of the Open Ephys GUI
+Copyright (C) 2013 Open Ephys
+
+------------------------------------------------------------------
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+This program 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 General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+*/
+
+/*
+This file references the CoreServices namespace header, in case a plugin type that usually doesn't
+access this namespace (FileSource, for example) needs, as a particular case, use its functionality
+*/
+
+#include "../../CoreServices.h"