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`.
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