Fire mu4e-index-updated-hook after any index operations, where anything changed
or not. Add a new variable `mu4e-index-update-status' which can be used to see
what changed (if anything) in the last indexing operation.
Keep mu4e-message-changed-hook in place; it only fires when a message
changed (and when it is update).
Remove the long-obsolete `mu4e-msg-changed-hook'.
Usurp more of the utils code than can be re-used without further dependencies in
helpers.
Split off specific parts in their own file.
After the helper/utils changes, update the rest of mu4e to take the changes into
account.
Dirk-Jan C. Binnema