ModelContainerWrapper: added size() method

src/modelcontainerwrapper.h

@@ -55,11 +55,16 @@ public:
 		return nullptr;
 	}
 
-	virtual bool is_container_empty()
+	virtual bool is_container_empty() const
 	{
 		return true;
 	}
 
+	virtual size_t size() const
+	{
+		return 0;
+	}
+
 	virtual void increment_iterator()
 	{
 	}
@@ -84,11 +89,16 @@ public:
 		iterator = container->end();
 	}
 
-	bool is_container_empty()
+	bool is_container_empty() const
 	{
 		return container->empty();
 	}
 
+	size_t size() const
+	{
+		return container->size();
+	}
+
 	void increment_iterator()
 	{
 		if( iterator == container->end() )