load xdg menu spec instead of just parsing .desktop files

[Antoine Martin]

The spec is ​here.
Maybe we can replace the whole logic of excluded apps with standard /menus/*.menu files? Ship our own in /usr/share/winswitch/ and use them as templates (copy them to the user's config directory?)
We should also parse the categories from '/usr/share/desktop-directories/' rather than implementing our own complicated logic for grouping into categories.

I wished there was some python library to load all these files into data structures already... And there is: ​http://www.freedesktop.org/wiki/Software/pyxdg (shame I only discovered it after writing so much code...)

See also #152

[Antoine Martin]

r4896 improves things