[IMP] convert.py xml <odoo> parsing and cleanups

- deprecate <url> tag
- deprecate multi levels name for <menuitem>
- <odoo> <openerp> <data> tags are now equivalent and may be nested, it means
  that the <data> tag is now optional and should only be used if the same xml
  file contains both update and noupdate records.

Only the base_menu.xml has been converted to the new format as an example.
Other files will be be progressively converted to avoid useless conflicts in
forward ports.