Runner issues with file selection

[peircej]

Several buglets:

• if you run a script and then, in Runner, move to a different item, then you can't ever get back to the Red Stop button (it gets disabled and never re-enabled when switching files). Fix so the red cross always kills the running process (only allowing 1 running script?) or track multiple processes (one per script?)
• there seem to be some issues as well with the wrong script sometimes running or sometimes no script being selected (if file is in the list and you press run from Coder it sometimes fails to select the right script)