Support multiple accounts for desktop version

[exap]

At the moment I can't really use multiple accounts, that is signing out from my current account and logging into a another account:

If I sign out from my current account, the keys will be deleted. So verifying a key becomes pointless because you'd have to verify a key everytime you sign into an account again.

Solution: Don't delete the keys when signing out. Only create keys on the first login from that device.

[richvdh]

dup #2320