Rework GPG behaviour:

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