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/DynamicProperty.cpp

@@ -30,6 +30,7 @@
 #include "Property.h"
 #include "PropertyContainer.h"
 #include "Application.h"
+#include "ExtensionContainer.h"
 #include <Base/Reader.h>
 #include <Base/Writer.h>
 #include <Base/Console.h>
@@ -69,6 +70,10 @@ Property *DynamicProperty::getPropertyByName(const char* name) const
     std::map<std::string,PropData>::const_iterator it = props.find(name);
     if (it != props.end())
         return it->second.property;
+    
+    if(this->pc->isDerivedFrom(App::ExtensionContainer::getClassTypeId()))
+        return static_cast<App::ExtensionContainer*>(this->pc)->ExtensionContainer::getPropertyByName(name);
+        
     return this->pc->PropertyContainer::getPropertyByName(name);
 }
 
@@ -113,6 +118,10 @@ const char* DynamicProperty::getPropertyName(const Property* prop) const
         if (it->second.property == prop)
             return it->first.c_str();
     }
+    
+    if(this->pc->isDerivedFrom(App::ExtensionContainer::getClassTypeId()))
+        return static_cast<App::ExtensionContainer*>(this->pc)->ExtensionContainer::getPropertyName(prop);
+        
     return this->pc->PropertyContainer::getPropertyName(prop);
 }
 
@@ -139,6 +148,10 @@ short DynamicProperty::getPropertyType(const Property* prop) const
             return attr;
         }
     }
+    
+    if(this->pc->isDerivedFrom(App::ExtensionContainer::getClassTypeId()))
+        return static_cast<App::ExtensionContainer*>(this->pc)->ExtensionContainer::getPropertyType(prop);
+        
     return this->pc->PropertyContainer::getPropertyType(prop);
 }
 
@@ -153,6 +166,10 @@ short DynamicProperty::getPropertyType(const char *name) const
             attr |= Prop_ReadOnly;
         return attr;
     }
+    
+    if(this->pc->isDerivedFrom(App::ExtensionContainer::getClassTypeId()))
+        return static_cast<App::ExtensionContainer*>(this->pc)->ExtensionContainer::getPropertyType(name);
+        
     return this->pc->PropertyContainer::getPropertyType(name);
 }
 
@@ -162,6 +179,10 @@ const char* DynamicProperty::getPropertyGroup(const Property* prop) const
         if (it->second.property == prop)
             return it->second.group.c_str();
     }
+    
+    if(this->pc->isDerivedFrom(App::ExtensionContainer::getClassTypeId()))
+        return static_cast<App::ExtensionContainer*>(this->pc)->ExtensionContainer::getPropertyGroup(prop);
+        
     return this->pc->PropertyContainer::getPropertyGroup(prop);
 }
 
@@ -170,6 +191,10 @@ const char* DynamicProperty::getPropertyGroup(const char *name) const
     std::map<std::string,PropData>::const_iterator it = props.find(name);
     if (it != props.end())
         return it->second.group.c_str();
+    
+    if(this->pc->isDerivedFrom(App::ExtensionContainer::getClassTypeId()))
+        return static_cast<App::ExtensionContainer*>(this->pc)->ExtensionContainer::getPropertyGroup(name);
+        
     return this->pc->PropertyContainer::getPropertyGroup(name);
 }
 
@@ -179,6 +204,10 @@ const char* DynamicProperty::getPropertyDocumentation(const Property* prop) cons
         if (it->second.property == prop)
             return it->second.doc.c_str();
     }
+    
+    if(this->pc->isDerivedFrom(App::ExtensionContainer::getClassTypeId()))
+        return static_cast<App::ExtensionContainer*>(this->pc)->ExtensionContainer::getPropertyDocumentation(prop);
+        
     return this->pc->PropertyContainer::getPropertyDocumentation(prop);
 }
 
@@ -187,6 +216,10 @@ const char* DynamicProperty::getPropertyDocumentation(const char *name) const
     std::map<std::string,PropData>::const_iterator it = props.find(name);
     if (it != props.end())
         return it->second.doc.c_str();
+    
+    if(this->pc->isDerivedFrom(App::ExtensionContainer::getClassTypeId()))
+        return static_cast<App::ExtensionContainer*>(this->pc)->ExtensionContainer::getPropertyDocumentation(name);
+        
     return this->pc->PropertyContainer::getPropertyDocumentation(name);
 }