Add a resync button that forces calendar refresh

Please add a resync button, so the calendar could be manually refreshed.
Useful when you added something to your google calendar from other source and you would like to see new changes in eOS calendar app.

---
Want to back this issue? **[Post a bounty on it!](https://www.bountysource.com/issues/78237008-add-a-resync-button-that-forces-calendar-refresh?utm_campaign=plugin&utm_content=tracker%2F60388692&utm_medium=issues&utm_source=github)** We accept bounties via [Bountysource](https://www.bountysource.com/?utm_campaign=plugin&utm_content=tracker%2F60388692&utm_medium=issues&utm_source=github).