* Move action handling code to rcmail class
* Add rcmail_action class
* Add action aliases
* Get rid of $OUTPUT global
* Move some methods from rcmail to rcmail_action
* PHP8 compat. fixes
* Add framework for testing actions
* Fix obvious code mistakes
When e.g. an encrypted message has one of it's sub-parts a part that is message/rfc822
part we need it's body and size, because it will be listed on the attachments list
and also can be downloaded.