[NoteUtils] Add DataBase64Encoded and ContentID fields to Attachment
KPIMTextEdit returns already base64-encoded images, so if I want to attach them, I have to decode it, just for KMime to encode them again. Instead, just make it possible to explicitly tell KMime that attachment is already base64-encoded. It is also useful to be able to set some unique identifier to an Attachment, just to be able to refer to it from the note (`contentID`). Right now we only have `label` field, which is not unique and is supposed to be user-facing.
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