Fix gpg decryption (#186)

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
Fix #186 part 2

This patch fixes the attachment listing when using `mu`.  For some
reason the '--decrypt' option among others where totally ignored when
listing the attachments.
Fix #186 Part 3

This patch fixes the attachment extraction (open, save, temp) when using
`mu4e`.  `mu4e` used to not notify the mu-server about the
mu4e-decryption-policy.  As a result mu-server did not decrypt the
attachments for extract, open, or temp.