Allow longer PIN length (better: allow full character set & arbitrary length passwords)

[xanoni]

Currently, CakeWallet only allows up to 6-digits in PINs. This is probably sufficient if we assume that the phone itself (and its lock code) can be trusted and won't be compromised, but that's a big and unnecessary assumption to make. Most other wallets that I have used allow arbitrary passwords with at least a double-digit number of characters.

Thus, I propose that users should be able to switch between two options:

1. PIN with up to 12 digits (default)
2. Arbitrary length password

People who don't like this don't have to use it.