Add "Delete All Attachments" action to context menu
Complementary to "Save All Attachments" this action can be applied to a single message currently displayed in message viewer as well as to a selection in message list.
It uses the same internal logic as "Save All Attachments", so if the user first saves all attachments and then deletes them, they should not lose any data.
A confirmation dialog is shown before the deletion, if there's at least one cryptographically signed message in the selection it will also warn about signature being invalidated by the operation, and if there's at least one encrypted message it informs the user that deleting attachments from encrypted messages is not supported.