Auto-detect changes to file and reload when window becomes active

[dstillman]

Given the template functions we're adding, keeping the code editor makes sense. But then we should make it easier to switch between another external editor, by auto-detecting if the file has changed. (Some editors prompt if the editor is dirty and the file was changed, but that seems unnecessary.)

[dstillman]

This should also let us remove the "Use External Editor" checkbox.