Use json object instead of QSet for sets

Summary:
This is a workaround for a crash that is an upstream Qt bug. Also JS object is more performant in checking whether something exists as an element in the set, as QSet is converted to a JS array when passed to QML.

Type: fix

Upstream: https://bugreports.qt.io/browse/QTBUG-131961

Test Plan: Verify pinning and unpinning events still work. Verify timeline gaps still work.

Reviewers: O1 the Kazv Project

Differential Revision: https://iron.lily-is.land/D208