Synchronize non background synchronized mailbox when they are opened

[ChristophWurst]

Feature Request

As of #3644 we don't sync every mailbox in background anymore. This means as you open those with the sync set to off you'll see an outdated representation of the mailbox. Only the background sync every 30s will eventually update the state.

In the case of those mailboxes we should, however, trigger the sync right away, so the view gets updated ASAP and the users sees the actual contents of that mailbox.

Ideally the sync state should be reflected in the UI so the user is aware what the application is doing.

Summary

Sync mailboxes without background sync when they are opened by the user.