Major rework of envelope address handling: a normalized version of the envelope addresses is maintained in the message context and message context is now also available during action execution.

Body: added test regarding empty bodies and fixed testsuite to flush the message context when the message is changed.

Testsuite: tested handling of teststuite envelope environment and fixed bugs in the envelope test in the process.

Added the concept of persistent tags and implemented imapflags extension for bare keep and fileinto commands (intermittent commit, not working properly yet).