Show: Tempovis stack support

Improvents:
* Allows using several tempovis instances in arbitrary order
* plugin system (SceneDetail), ease of extending
* Mild restore feature - more respect for changes made by user
* workbench switching support
* undo saveable changes for the time of writing a file

Regressions:
* Removed support for pickling (with hopes to reintroduce later)

src/Mod/Show/TVObserver.py

@@ -0,0 +1,48 @@
+#/***************************************************************************
+# *   Copyright (c) Victor Titov (DeepSOIC)                                 *
+# *                                           (vv.titov@gmail.com) 2019     *
+# *                                                                         *
+# *   This file is part of the FreeCAD CAx development system.              *
+# *                                                                         *
+# *   This library is free software; you can redistribute it and/or         *
+# *   modify it under the terms of the GNU Library 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 Library General Public License for more details.                  *
+# *                                                                         *
+# *   You should have received a copy of the GNU Library General Public     *
+# *   License along with this library; see the file COPYING.LIB. If not,    *
+# *   write to the Free Software Foundation, Inc., 59 Temple Place,         *
+# *   Suite 330, Boston, MA  02111-1307, USA                                *
+# *                                                                         *
+# ***************************************************************************/
+
+from . import TVStack
+import FreeCAD
+
+class TVObserver(object):
+    def __init__(self):
+        FreeCAD.addDocumentObserver(self)
+    
+    def stop(self):
+        FreeCAD.removeDocumentObserver(self)
+    
+    def slotStartSaveDocument(self, doc, filepath):
+        TVStack._slotStartSaveDocument(doc)
+    
+    def slotFinishSaveDocument(self, doc, filepath):
+        TVStack._slotFinishSaveDocument(doc)
+        
+    def slotDeletedDocument(self, doc):
+        from . import TVStack
+        TVStack._slotDeletedDocument(doc)
+
+#handle module reload
+if 'observer_singleton' in vars():
+    observer_singleton.stop()
+
+observer_singleton = TVObserver()