[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.