This commit fixes several issues which together could lead to errors
which result in event duplication in some cases.
- sync anchor timestamp is now stored exclusively in the notebook
as the syncDate property, to avoid atomicity issues
- sync anchor token is now stored exclusively in the notebook
as a custom property (instead of account setting) to avoid
- detection of server-side sync token invalidation has been
improved, allowing the sync envelope to be tweaked appropriately
- detection of events copied between calendars via Google web UI
has been added, to ensure we don't attempt to store multiple
events with the same UID (which could trigger a clean-sync cycle).
It also bumps the plugin version to ensure that a clean sync will
be triggered on the first run, to clean up any duplications which
may have previously occurred.
Contributes to JB#47783