Allow different master passwords

[607011]

All instances of this app running on different desktop computer or mobile devices should be able to sync their common data by using their own master password.

This could be achieved by public-key cryptography.

[Wikinaut]

Implementation idea:

@ola-ct you could think of borrowing the "key slot" idea from dm-crypt, which basically allows for a certain set of n passphrases https://de.wikipedia.org/wiki/Dm-crypt .

[amenk]

ACK