src/extension.js: Add a proxy object to the NetworkManager DBus
object, to query the state of the connection and connect to changes.
We don't try to query the data if we have no connection, and we
immediately query the data if we get a connection. This results in
better behaviour from the extension, specially after resuming from
suspend-to-ram: Before, the extension would try to get the data almost
immediately after resuming, but usually the connection was not
available yet. Now we wait to get a connect signal before getting the
data.