GNOME Shell emits a warning when "url" is absent in metadata.json,
adding it costs nothing. Links to the GIT repository because I
found no better page to reference.
Introduce a translation domain, gnome-shell-extensions, and use it
in extensions that have user-facing strings, instead of abusing
of translations provided by gnome-shell.
Also includes an example translation for italian, and bump the
required version for xrandr-indicator.
Depends on GNOME bug 621017.
