Don't ask for save when quitting in a clean state (e.g. right after a previous save)

[xparq]

I don't know if MuLab does or can maintain a "clean/dirty" edit state, but if it's already available, or possible to do, it could mean one less nuisance.

If there's nothing to save, the quit confirmation should just be: [Quit | Cancel].
(And if there is some, then: [Quit+Save | Quit | Cancel] --> see #39 why this ordering would be preferable.)