search for: notify_register

...took some inspiration from dovecot-core/src/plugins/notify/notify-storage.c -- this requires the use of struct mail_private to be able to override the vfunc for mail_update_keywords, which I actually do not like. However, when I tried to make this patch base on the use of the notify plugin (via notify_register), I ran into the issue that the transaction is partially freed before the transaction_commit event from notify is emitted, which is unfortunate and breaks at least the mailtrain backend (which I use). This is in theory fixable, but I wanted to keep the patch as small as possible, as I have no...