Allow an event updates on iTip request without SEQUENCE bump (Bifrost#T27883)

We're detecting if something changed in description, title, url,
location or attendees. These are properties that could be changed
by the organizer without SEQUENCE bump. If that is detected
we display "Update in my calendar" button.

TODO: display a diff of changes, so user can take a decission

if he want's to overwrite his copy of the event.