Support xdg-user-dirs in all GNOME modules

[Luca Ferretti <elle uca libero it>]

Some GNOME modules didn't switched to xdg-user-dirs and they are using a
raw "$HOME/Desktop" as path to desktop dir. This will make them fails
when the user switched to new approach.

Examples:
  http://bugs.gnome.org/453115 - bug-buddy
  http://bugs.gnome.org/455204 - gnome-screenshot

Both program try to preset $HOME/Desktop as directory to save shots or
bug report details; if you switched to xdg-user-dirs and you no longer
have $HOME/Desktop (i.e. you are not using English as your language),
the GtkFileChooserButton shows an error message related to missing
directory.

Are bug-buddy and gnome-screenshot the only modules that needs to be
updated? Could someone review proposed patches?