Watch the ledger device connection and warn if lost

[monikasmolarek]

Currently, Wallet doesn’t notice when the device is disconnected until the very last step of a device-related process.

The icon indicator also stays green showing the user that the ledger is still correctly connected, while in reality it might be unplugged.

To cover:

• Verify the behavior after the Spacemesh App is closed on the ledger while it's still connected to the lite wallet app.
• Verify the behavior after the ledger is locked (i.e. being idle) while it's still connected to the lite wallet app.
• Verify the behavior after the ledger is unplugged while it's still connected to the lite wallet app.