Improve settings backup dialog

[fwdekker]

In #602 / #604 I implemented a dialog for creating a backup of one's settings. I did not implement everything I wanted to because I wanted to get the fixes out there as fast as possible.

There are a few more things I think should be added to make this feature actually complete:

• Add a banner to the settings window notifying the user that their settings will not be saved due to settings corruption.
• Add an option to the dialog to allow the user to import a backup.
• Make the dialog accessible from the general settings menu, not just from the occasional notification.