Make code a bit more pythonic.

Fix some strange encoding error.

Backport some fixes to zeroconf: don't fail on special characters, don't allow gpg usage when it is not secure or even possible
Be more verbose when there is a key missmatch.

Mind that presence in zeroconf are not signed, gpg keys have to be assigned manually therefore.

Do not try to send encrypted messages when we don't use GPG, though it is enabled per user.

Only encrypt when the receiver is trusted. You have to sign its key or it has to be signed by someone you trust. Fixes #109

Make checkbox insensitive when GPG is disabled on an account (or no passphrase given).

Auto assign trusted keys on received presence. Deny encryption on missmatch of assigned key and signing key. Fixes #3376

Do not disable encrypted when receiving an unencrypted message. Print whether a received message was encrypted or not.

TODO: Remove togglebutton, its useless now

ability to block / unblock a transport. Remove log on / log off (it's the same as send_custom_status)

Refactor the way we decide whether GPG is usable or not: Ability to change keys when no valid passphrase could be entered.
See #1210. Workaround for unavailable keys.

Remove useless except block: import of GnuPGInterface is the same on Debian and non Debian systems.

Do not fail when no key is selected.