[UX] Warn if there are unsaved requests when quitting Bruno

[grubersjoe]

Situation: I sometimes edit my requests but forget to save them. When I quit Bruno with cmd+q etc. my changes will be lost without a warning. I'm also mostly used to applications to just remember my input or at least ask for confirmation before continuing with a destructive operation.

I've lost my changes several times already and I think UX could be improved a lot here by simply warning the user before quitting when there are unsaved files.

For closing an unsaved tab this already is implemented. So this could probably be reused for the whole application as well.