Track internet connectivity using NMClient

If network connection isn't up when the extension starts it will show no
weather information for long period of time. It is fairly simple to
avoid that issue if using networkmanager signal for connection state to
reload weather information when connection comes up.

Signed-off-by: Pauli Nieminen <suokkos@gmail.com>
Reload opens network chooser menu if no connection.

If no network connection is running user probably wants reload to
propose connecting to internet. With that assumption we open network
chooser menu.

This change uses shell panel internal interfaces so it may break between
versions. I only tested with gnome 3.8. Gnome 3.10 seems to be about
rewrite panel menus to dialogs that most likely requires changes to
extensions too.

https://wiki.gnome.org/GnomeShell/Design/Guidelines/SystemStatus/
https://mail.gnome.org/archives/commits-list/2013-July/msg07691.html

Signed-off-by: Pauli Nieminen <suokkos@gmail.com>