Merge changes from topic 'bouncing'

* changes:
  tests: fix copyright header
  ComposeWidget: let read-only message take all available vertical space
  Strip the mailbox header when uploading messages from files
  bounce: Add the Resent-* headers
  Refactoring: Move Message-ID generating to a reusable place
  UI support for message bouncing ("open in composer")
  GUI: Show when we cannot send yet/anymore
  New composer backend that provides read access to an existing message
  Prepare ComposeWidget for other implementations of AbstractComposer
  Fix MessageComposer's memory management
  composer: dependency injection: MessageComposer -> ComposeWidget
  composer: abstract interface for common MessageComposer's methods
  composer: add the `override` keyword
  QAIM: access to IMAP URLs for IMAP messages and their parts