While auto-saving works to some degree, I find it much less useful than it could be when I want to depend on it myself. This patch implements a couple of improvements.
* Change save interval unit from minutes to seconds. For my own work, I'd like to set the interval very low, because Blender crashes a lot during development and basic testing.
* Don't auto-save when the current state of the file is already saved (either as auto-save or normal save).
* New "Recover Last Auto Save" operator that finds the most recent auto-saved file and opens it. That's much more convenient when recovering after a crash compared to picking the right file in the `/tmp/` directory.
On top of that, to unify things a bit, it sounds reasonable to get rid of `quit.blend` and use the auto-save system for that as well. `quit.blend` is just "auto saving" as well really. So instead of writing `quit.blend` when Blender quits, we just call `wm_autosave_write` instead. That might also help protect the user when quitting multiple instances of Blender, since auto-saving encodes the process id in the file name. The "Recover Last Session" operator on the splash screen should then just do what the new "Recover Last Auto Save" operator is doing in this patch (find the latest auto-save).
Does that sound like a reasonable change?