Check we're not presenting multiple modal dialogs at once on startup

[dae]

The sync on startup, add-on update notification, and Anki update notification dialogs are all modal. We should make sure we don't present more than one at once, perhaps by only triggering the update checks after an initial sync, and only one after the other.

https://forums.ankiweb.net/t/dialogue-box-collision-on-startup/21724