Trezor firmware versioning

[ImplOfAnImpl]

• The wallets now obtain the Trezor firmware version and refuse to continue if its major component doesn't match the expected value.
  Node-gui displays the firmware version in its status bar; for wallet-cli, it is printed by wallet-info.

• A couple minor issues were fixed:

  • node-gui didn't handle multiple connected devices well, producing an obscure error like "No wallet opened". Now it prints a meaningful error at least, suggesting to leave only one device connected and retry;
  • wallet-cli's Trezor device selection was broken, failing with the "File already exists" error, because we didn't delete the temporarily created wallet file if MultipleAvailableTrezorDevices was being returned (in create_wallet we were deleting it only on error and in recover_wallet we weren't deleting it at all).

• A Trezor-specific readme file was added with some security-related caveats and build instructions.

• Minor cleanup of existing readme files.