Show a message when opening the wallet while offline

[mvaivre]

As of today, the data is simply not loaded (no addresses, amounts etc.) but no error are shown.
Also, "refreshing" (using the dedicated header button) after getting back online doesn't work.

As a first step, when offline, we could show an error snackbar and replace the refresh icon with a "offline" icon.

In the future, we could cache previously loaded data to populate the UI, even when offline.

[lf94]

Should it be clear that the data presented is old?

[mvaivre]

Yes. An easy way could be to add a little state icon to the refresh button:

With an explanation on hover: "(Not) Synchronized"