[nemo-qml-plugin-calendar] Expose sync failures to events.
Based on volatile properties of KCalCore (see kcalcore!19 (merged)), read sync failures and export them to QML.
Every sync plugin can set (and unset) this
SYNC-FAILURE volatile property for events that have issues to be propagated to or from the server. See for instance how I'm doing it in CalDAV: dcaliste/buteo-sync-plugin-caldav@ad1e6fca
Like that a user can be warn if an event has issues (see the contributor-failures branch in jolla-calendar).