"Exit" confirmation modal dialog

[LyubomirT]

To prevent users from accidentally clicking the "Exit" button, we could add a confirmation modal that will appear after the button is clicked. That modal will ask the user if they really want to exit or not. This can be built into index.html following the same structure as other modals.

Hint:
This will require modifying the modals in index.html, and the exit trigger in renderer.js.