Add UI for managing database snapshots

[fuzyll]

What is the feature you'd like to have? A user requested a "save with backup" option today because of bad experiences they've had with tools like IDA in the past corrupting on save and losing work. Having an option for automatic backups on save is a good idea for people who are paranoid about losing work (and also shouldn't be too tedious for most binaries now that we've done our performance improvements for database load/save).

Is your feature request related to a problem? It's theoretically possible that, on save, a database could be corrupted. Recovering from this should be fairly easy, but depending on the nature of the corruption, could be incredibly difficult.

Are any alternative solutions acceptable? Having a backup copy automatically saved that you could return to is the way other tools like IDA handle this problem. There are probably alternatives that are acceptable, but I believe they'd be more work than simply doing this.

[fuzyll]

We discussed this and I don't think this feature should be necessary. The way we do our databases (which is significantly different from IDA's) should mean the only thing that could happen is a failed snapshot save.

But, what we should do is provide a better interface for viewing and managing snapshots within the database so that a user can revert a corrupted or otherwise unwanted snapshot more easily.

[fuzyll]

This needs to also show the description once #3367 is implemented.

[CouleeApps]

This has been partially implemented in 3.5.4321-dev with the addition of the Database Viewer (Debug) plugin. Non-debug support is not planned at this time so I'm leaving this open and unassigned.