`Unsynced notes` alert should be dismissed/updated if the connection was restored.

[Same for iOS.](https://github.com/Automattic/simplenote-ios/issues/1254)

### Nice to have
If the user sees `Unsynced notes` alert, but the connection was restored and notes were synced, the alert should be updated/replaced with the other one saying that all notes were synced, and it's safe to log out now / cancel. This is so in Electron.

### Why
If the user taps Delete Notes (while notes are already synced, but the user doesn’t know this) the app just logs out. While having only the phone, it’s not clear if the note was deleted or not, and if the connection was restored or not. (in reality, the notes do not get deleted and the user logs out).

Start by locating the macOS code that presents the `Unsynced notes` alert and handles restored connectivity, then compare the expected behavior with the linked iOS issue and the Electron implementation mentioned here. Done means the alert changes to the safe-to-log-out state after notes finish syncing, so Delete Notes no longer leaves the user uncertain.

Written by the indexing model from the issue text.