Port of Menu & Menu editor to ElementTree

And also some refactoring of the Menu parsing methods: parsing is now done by the xdg.Menu.Parser.parse() method. The xdg.Menu.parse() function is left as a shortcut.
All Menu classes are freed from the parsing job.

There are no unit tests for the MenuEditor class... Do you have some code I could use to test against (and turn to proper unit test while I'm at it) ?