Make python2 explicit

Nowadays we really need to explicitly state which python we require.
Fallback from Ubuntu to generic GNOME app

Use GtkStatusIcon() for fallback from Ubuntu messaging menu. Previously I
was using the Ubuntu application indicator for fallback (which I
believed wraps GtkStatusIcon anyhow). So now this app should work in
any generic GNOME environment, not just any Ubuntu system.

I have tested this port running on a Arch GNOME 3/Shell system.