Decouple QofBook creation from QofSession.

So that we don't create two books when loading a session.
Step 1 to not having a dirty book when we think we should have no
book at all.

bindings/python/gnucash_core.py

@@ -103,7 +103,7 @@ class Session(GnuCashCoreClass):
         you don't need to cleanup and call end() and destroy(), that is handled
         for you, and the exception is raised.
         """
-        GnuCashCoreClass.__init__(self, instance=instance)
+        GnuCashCoreClass.__init__(self, Book())
         if book_uri is not None:
             try:
                 self.begin(book_uri, ignore_lock, is_new, force_new)