Unset extensibles on larger objects when they destruct

Some complex extensibles require the full object to still be alive in
their destructors to keep proper bookkeeping

src/extensible.cpp

@@ -23,6 +23,11 @@ ExtensibleBase::~ExtensibleBase()
 }
 
 Extensible::~Extensible()
+{
+	UnsetExtensibles();
+}
+
+void Extensible::UnsetExtensibles()
 {
 	while (!extension_items.empty())
 		(*extension_items.begin())->Unset(this);