Update various places for what we can do with C++20:
- ends_with / starts_with
- std::to_array
- using instead of typedef
- designated initializers
- "[[maybe_unused]]" instead of G_GNUC_UNUSED
Also remove some unnecessary 'static'
Do not consider calender-invitations "attachments"; do mark as
"calendar". Do recognize application/ics as calendar messages.
Update mime-object to expose a message part's disposition.
Change the "is-attachment" heuristic to include inline parts if they
have a filename parameter in their content-disposition.
Note that this doesn't change things radically; the delta is +69 and
-202 for ~6500 attachments.
Clean up the implementation at bit, and filter out 'fake' message-ids,
such as the ones from protonmail.
Update documentation.
Add Mu::Message::thread_id().
This fixes#2312.
