stops running process when quitting editor

I noticed a bug where I could close Godot while I was running a project.

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.
Merge pull request #324 from adolson/close_running_program_on_editor_quit

stops running process when quitting editor