Revert "Do not handle signature parts after verification"

This reverts commit 6e9b9ad2d0c81dc0ad8f7a5c1b4981742d9fee22.

Unfortunately the reverted commit breaks the Signature field for
encrypted and, at the same time, signed messages.

TODO: details button in the Signatures field does not work for such
cases because the signature is encrypted.

Conflicts:
	lib/mu-msg-part.c
Fix signatures' detail box for encrypted messages

In the case of encrypted and signed messages the signature field's
details box did not work due to missing flags to the mu verify command.
This commit fixes this issue.