Base: Observer - fix template instantiation

===========================================

Merit goes to Werner for figuring out the issue and proposing a MSVC/MSYS2 viable solution:
https://github.com/FreeCAD/FreeCAD/pull/8781#issuecomment-1464898566

src/Base/CMakeLists.txt

@@ -231,6 +231,7 @@ SET(FreeCADBase_CPP_SRCS
     MatrixPyImp.cpp
     MemDebug.cpp
     Mutex.cpp
+    Observer.cpp
     Parameter.xsd
     Parameter.cpp
     ParameterPy.cpp

src/Base/Observer.cpp

@@ -0,0 +1,34 @@
+/***************************************************************************
+ *   Copyright (c) 2023 Abdullah Tahiri <abdullah.tahiri.yo@gmail.com>     *
+ *                                                                         *
+ *   This file is part of the FreeCAD CAx development system.              *
+ *                                                                         *
+ *   This program is free software; you can redistribute it and/or modify  *
+ *   it under the terms of the GNU Library General Public License (LGPL)   *
+ *   as published by the Free Software Foundation; either version 2 of     *
+ *   the License, or (at your option) any later version.                   *
+ *   for detail see the LICENCE text file.                                 *
+ *                                                                         *
+ *   FreeCAD 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 Library General Public License for more details.                  *
+ *                                                                         *
+ *   You should have received a copy of the GNU Library General Public     *
+ *   License along with FreeCAD; if not, write to the Free Software        *
+ *   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  *
+ *   USA                                                                   *
+ *                                                                         *
+ ***************************************************************************/
+
+#include "PreCompiled.h"
+
+#include "Observer.h"
+
+
+namespace Base {
+
+template class BaseExport Observer<const char*>;
+template class BaseExport Subject<const char*>;
+
+} //namespace Base

src/Base/Observer.h

@@ -216,16 +216,17 @@ protected:
   std::set<Observer <_MessageType> *> _ObserverSet;
 };
 
-#if defined (FC_OS_WIN32) || defined(FC_OS_CYGWIN)
-#   ifdef FCBase
-template class BaseExport Observer<const char*>;
-template class BaseExport Subject<const char*>;
-#   else
-extern template class BaseExport Observer<const char*>;
-extern template class BaseExport Subject<const char*>;
-#   endif
+// Workaround for MSVC
+#ifdef FreeCADBase_EXPORTS
+#  define Base_EXPORT
+#else
+#  define Base_EXPORT  BaseExport
 #endif
 
+extern template class Base_EXPORT Observer<const char*>;
+extern template class Base_EXPORT Subject<const char*>;
+
+
 } //namespace Base
 
 

src/Gui/QtAll.h

@@ -191,6 +191,7 @@
 #include <QStackedWidget>
 #include <QTableWidgetItem>
 #include <QWidget>
+#include <QWidgetAction>
 
 // QtXML
 #include <QDomDocument>