Gasetta: support of various XDG specifications

[florian]

FreeDesktop has specifications about various desktop-related stuff (like directories to store configurations).

Those specs should be respected on XDG-compliant desktops (almost all free desktops), by example through the PyXDG module, without being a dependency on legacy system likes the Microsoft Windows one.

For Gasetta, the main item to change is the location of the configuration file. We can also use it to add a recent document support.