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/SceneDetail.py

@@ -0,0 +1,78 @@
+#/***************************************************************************
+# *   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                                *
+# *                                                                         *
+# ***************************************************************************/
+
+class SceneDetail(object):
+    """SceneDetail class: abstract class for tempovis scene save/restore plug-in. An implementation must provide:
+    * data storage (as "data" attribute of the object)
+    * constructor (preferably, with value for stored data as optional argument)
+    * methods to apply values to actual scene (apply_data), 
+    * ...and to read out the state of the detail in the actual scene (scene_value)
+    * keying, for identifying two detail instances that affect the exact same thing
+    * class_id string, which is required for keying
+    * copying
+    * test for equality, that checks if .data attributes of two SceneDetail instances are equal
+    * info on if the modification affects what is saved to disk, and thus should be undone temporarily for file writing.
+    """
+    class_id = ''
+    
+    data = None
+    doc = None
+    
+    ## Storage field for TV. Mild restore means that the saved state won't be restored 
+    # if the current state differs from the state requested via last TempoVis.modify(...) call. 
+    # This is a default, it may be changed per-instance. 
+    mild_restore = False 
+ 
+    def set_doc(self, doc):
+        self.doc = doc
+        
+ # <interface>
+    key = None #a string or something alike to use to store/find the entry in TempoVis. For example, a string "{object_name}.{property_name}". 
+    affects_persistence = False #True indicate that the changes will be recorded if the doc is saved, and that this detail should be restored for saving
+        
+    def scene_value(self):
+        """scene_value(): returns the value from the scene"""
+        raise NotImplementedError()
+    
+    def apply_data(self, val):
+        """apply a value to scene"""
+        raise NotImplementedError()
+    
+    ## Equality test. Override if data attributes can't be directly compared
+    def __eq__(self, other):
+        if isinstance(other, self.__class__):
+            return self.data == other.data and self.data is not None
+        else:
+            raise TypeError('{self} can\'t be compared with {other}'
+                             .format(self= repr(self), other= repr(other)))
+  # </interface>
+  
+  # <utility>
+    @property
+    def full_key(self):
+        return (self.class_id, self.doc.Name if self.doc else None, self.key)
+    
+
+    def __ne__(self, other):
+        return not self.__eq__(other)
+