After a multipart/encrypted part gets decrypted the result is usually a
`multipart/mixed` part (see enigmail).
Before this commit mime multiparts where handled only by
`g_mime_message_foreach`. As a result the decrypted mime multiparts
where not processed.
This patch handles mime multiparts explicitly by removing the
`g_mime_message_foreach` invocations. This might come at the cost of
reduced maintainability, in the case of radical gmime changes. However,
gmime is pretty stable and that scenario is highly unlikely.
TODO: After decryption make any attachments available
29e53d2eba