notebook: Notebook.get_page(): Cross-reference page to owning notebook.

This allows page to know filesystem layout and gazillion of other things.
Specific motivation for adding this is to allow generate/resolve
filesystem-based inter-page links for Markdown format.

Signed-off-by: Paul Sokolovsky <pfalcon@users.sourceforge.net>

zim/notebook/notebook.py

@@ -426,6 +426,7 @@ def get_page(self, path):
 			folder = self.layout.get_attachments_folder(path)
 			format = self.layout.get_format(file)
 			page = Page(path, False, file, folder, format)
+			page.notebook = self
 			try:
 				indexpath = self.pages.lookup_by_pagename(path)
 			except IndexNotFoundError: