Show a clear indication when offline

Polari is almost entirely useless without an internet connection, yet our current indication that connectivity has been lost is so subtle that it is easily missed.

To address this, try harder to get the user's attention by popping up an info bar that outlines the problem and offers a "Network Settings" button to help them resolve the issue:

See https://bugzilla.gnome.org/show_bug.cgi?id=760833 for the original discussion.