stops running process when quitting editor

I noticed a bug where I could close Godot while I was running a project, and the project would continue to run.

If the program was left running and then ran into an error, or if an error was caught by the debugger and then I quit Godot, I could no longer close the running project without manually killing it.
This fixes that problem by essentially hitting the Stop button automatically just prior to quitting the editor.