Maybe add mnemonic (BIP39) support for iota seeds

[joshirio]

Open to discuss if implementing mnemonic seeds make sense.

We could use the already existing python iota mnemonic lib:
https://github.com/mlouielu/iota-mnemonic

The downside is that there's not a standard for IOTA seeds yet, so this could cause incompatibilities with other multisig softwares. IOTAcooler could include a converter (accessible through the tools menu) to solve this.

But since the user is highly unlikely to type the seed by keyboard for each transaction, we can assume that most users copy and paste the seed (from keepass, for example) on the offline singing device anyway, so the benefits are limited.