Fix #280

Since `parent` is not really used as a parent, I use it as the last
visited encrypted part while going down the parts-tree.

At the decryption of a part (`mu_msg_crypto_decrypt_part`) I check,
through the GMimeDecryptResult, for signatures (`check_decrypt_result`)
and add them to the part (`tag_with_sig_status`).  Any nested parts hold
that encrypted part as their parent.  Finally at `handle_part`, for each
part I check if it a descendent of an encrypted part.  If so, I proceed
checking for signatures and adding them to the `msgpart`.

lib/mu-msg-priv.h

@@ -115,10 +115,9 @@ typedef char* (*MuMsgPartPasswordFunc)   (const char *user_id, const char *promp
  *
  * @return a status report object, free with mu_msg_part_sig_status_report_destroy
  */
-MuMsgPartSigStatusReport* mu_msg_crypto_verify_part (GMimeMultipartSigned *sig,
-						     MuMsgOptions opts,
-						     GError **err)
-                                         G_GNUC_MALLOC G_GNUC_WARN_UNUSED_RESULT;
+void mu_msg_crypto_verify_part (GMimeMultipartSigned *sig,
+                                MuMsgOptions opts,
+                                GError **err);
 
 /**
  * decrypt the given encrypted mime multipart