Edit event detail for one occurence

## Problem

Currently, Calendar allows removal of one occurrence or all occurrences. However, Calendar does not allow changing the details of an event per occurrence, nor is there any indication that changing details of an event will change all occurrences. Instead, the user is left to guess and try, which is a bad experience.
_Clicking on 'save changes' will put the change into effect for all occurrences_

## Proposal

Add a button called 'save for this occurrence' and change the current button into 'save for all occurrences' if the event is recurring.

## Prior Art

_Same feature in Google Calendar -- too cumbersome to click save and then be required to select something again_

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