Add Offline Support

[marbetschar]

Prerequisites

• [x] I have searched open and closed issues for duplicates.

Feature

It would be awesome to have offline support in the Calendar app. Right now, if the app is started while we do not have any internet connection it does not display any event.

Describe the solution you'd like

Show all known events in the Calendar while in offline mode and sync possible modifications as soon as we are back online.

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[mcclurgm]

At least while it still requires a connection, it would be nice if it could show some sort of error message when it can't connect. As it is, it just displays an empty calendar when not connected, which is not helpful and potentially confusing.

[fabiozaramella]

It seems that starting Calendar offline doesn't show previously imported online events yes, but only at first. In fact, after a while they show up properly, even offline, it's just very slow.

EDIT: I thinks the reason is that currently the wait_for_connected_seconds parameter when calling ECal.Client.connect is set to wait 30 seconds, which is far to much, I think we can safely set it to -1 not to wait and then assure Calendar is synced when connected back to the Internet.