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.
Fix the behavior of the PopupMenu after the conversion of
St.Clickable to St.Button, and fix the invocation of ShellApp.open_new_window(),
which now accepts a workspace index as a number (as a side effect,
"New window" means one the current workspace now).
Giovanni Campagna