Extension: Fix order-of-initialisation crash

FreeCADs property system utilises some pointer math to calculate the offset between
property and base class. Due to virtual inheritance of th ePropertyContainer the memory
layout has been changed to rather random, which has lead to crashes dependend on the
order of object initialisation.

The solution is to not make PropertyContaner virtual but a class below, Base::Persitance.
Then the memory layout is random for Persistance, but it is perfectly aligned for the
base class chains from PropertyContainer onwards as well as from Extension onwards.
Hence the proeprty system was changed to take the offset always from those two.

src/App/Extension.cpp

@@ -34,19 +34,15 @@
 #include <Base/Console.h>
 #include <Base/PyObjectBase.h>
  
-/* We do not use a standart property macro for type initiation. The reason is that we want to expose all property functions, 
- * to allow the derived classes to access the private property data, but we do not want to have our
- * property data a reference to the parent data. That is because the extension is used in  a multi 
- * inheritance way, and hence our propertydata partent data would point to the same property data
- * as any other parent of the inherited class. It makes more sense to create a total unrelated line
- * of property datas which are added as additional parent to the extended class.
+/* We do not use a standart property macro for type initiation. The reason is that we have the first
+ * PropertyData in the extension chain, there is no parent property data. 
  */
 TYPESYSTEM_SOURCE_P(App::Extension);
-const App::PropertyData * App::Extension::getPropertyDataPtr(void){return &propertyData;}
-const App::PropertyData & App::Extension::getPropertyData(void) const{return propertyData;}
+const App::PropertyData * App::Extension::extensionGetPropertyDataPtr(void){return &propertyData;}
+const App::PropertyData & App::Extension::extensionGetPropertyData(void) const{return propertyData;}
 App::PropertyData App::Extension::propertyData;
 void App::Extension::init(void){
-  initSubclass(App::Extension::classTypeId, "App::Extension" , "App::PropertyContainer", &(App::Extension::create) );
+  initSubclass(App::Extension::classTypeId, "App::Extension" , "Base::Persistence", &(App::Extension::create) );
 }
 
 using namespace App;
@@ -57,7 +53,6 @@ Extension::Extension()
 
 Extension::~Extension()
 {
-    Base::Console().Message("Delete extension\n");
     if (!ExtensionPythonObject.is(Py::_None())){
         // Remark: The API of Py::Object has been changed to set whether the wrapper owns the passed
         // Python object or not. In the constructor we forced the wrapper to own the object so we need
@@ -82,6 +77,13 @@ void Extension::initExtension(ExtensionContainer* obj) {
     if(m_extensionType.isBad())
         throw Base::Exception("Extension: Extension type not set");
  
+    //all properties are initialised without PropertyContainer father. Now that we know it we can
+    //finaly finsih the property initialisation
+    std::vector<Property*> list;
+    extensionGetPropertyData().getPropertyList(this, list);
+    for(Property* prop : list)
+        prop->setContainer(obj);
+    
     m_base = obj;
     m_base->registerExtension( m_extensionType, this );
 }
@@ -106,8 +108,62 @@ const char* Extension::name() {
         return std::string().c_str();
 }
 
+
+
+Property* Extension::extensionGetPropertyByName(const char* name) const {
+    
+    return extensionGetPropertyData().getPropertyByName(this, name);
+}
+
+short int Extension::extensionGetPropertyType(const Property* prop) const {
+    
+    return extensionGetPropertyData().getType(this, prop);
+}
+
+short int Extension::extensionGetPropertyType(const char* name) const {
+    
+    return extensionGetPropertyData().getType(this, name);
+}
+
+const char* Extension::extensionGetPropertyName(const Property* prop) const {
+    
+    return extensionGetPropertyData().getName(this,prop);
+}
+
+const char* Extension::extensionGetPropertyGroup(const Property* prop) const {
+    
+    return extensionGetPropertyData().getGroup(this,prop);
+}
+
+const char* Extension::extensionGetPropertyGroup(const char* name) const {
+        
+    return extensionGetPropertyData().getGroup(this,name);
+}
+
+
+const char* Extension::extensionGetPropertyDocumentation(const Property* prop) const {
+    
+    return extensionGetPropertyData().getDocumentation(this, prop);
+}
+
+const char* Extension::extensionGetPropertyDocumentation(const char* name) const {
+    
+    return extensionGetPropertyData().getDocumentation(this, name);
+}
+
+void Extension::extensionGetPropertyList(std::vector< Property* >& List) const {
+    
+    extensionGetPropertyData().getPropertyList(this, List);
+}
+
+void Extension::extensionGetPropertyMap(std::map< std::string, Property* >& Map) const {
+
+    extensionGetPropertyData().getPropertyMap(this, Map);
+}
+
+
 namespace App {
-PROPERTY_SOURCE_TEMPLATE(App::ExtensionPython, App::ExtensionPython::Inherited)
+EXTENSION_PROPERTY_SOURCE_TEMPLATE(App::ExtensionPython, App::ExtensionPython::Inherited)
 
 // explicit template instantiation
 template class AppExport ExtensionPythonT<Extension>;