event-popover: Rework UI and code

Depends on !418 (merged)

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This builds on top of the existing UI/code, while adding a few improvements and behavioral changes:

• The popover displays the changes-prevent icon when the even is read-only.
• Each section is properly separated with GtkSeparators, with equal amount of top and bottom margins.
• Location and Meetings section are mutually exclusive; only one is shown.
• When an event has no event note, the popover will always display that there's no description.
• GcalMeetingRow has been ported to GtkListBoxRow.
• The action button adapts its icon and tooltip text depending on the event permission.

Screenshots