Allow deletion of attachments from message structure view

Deletion of attachments (and other message parts) is only allowed
one at a time. Message parts inside encrypted message parts cannot
be deleted.

This (re-)adds the "Delete Attachment" action to the context menu
of the message structure view. It was removed 3 years ago because
it always crashed (because it tried to work with a copy of the node
tree of the original message).

Sponsored by: Intevation GmbH

CCBUG: 287177
BUG: 436089
FIXED-IN: 5.18.0
2 jobs for master in 8 minutes and 57 seconds (queued for 18 minutes and 23 seconds)
Status Name Job ID Coverage
  Build
canceled build_clazy_clang_tidy #103007

canceled linux kf5-qt5 qt5.15 #103006

00:08:57