Prompt user to terminate if quitting while project is running

[hhhapz]

Here's the prompt message:

One thing that is technically problematic is if you make changes while the project is running and don't have 'save before running' checked, you could technically lose unsaved progress if you click to terminate.

Proposal: https://github.com/godotengine/godot-proposals/issues/6915

CC: @Calinou