Download updates in the background

[viktorgullmark]

When the notification shows, we should automatically start downloading an update, so we can trigger the install directly from the link in the image above.

Right now it works like this:

1. Automatically checks for updates
2. Shows notification if an update is available
3. When you restart, it starts downloading in the background
4. When the download is complete, you get a prompt again

This is problematic for people with slower internet connections, since the user won't get feedback about the update being installed or that the download is in progress.