add Quit command to Assistant's remote control API

There's no documented way to quit the Assistant via its remote control API. I'm leveraging the tool as an external documentation browser via QProcess, and can only terminate it with a QProcess::terminate() or by sending it a SIGHUP when my application exits. I don't know what (future) side-effects this may have, and it always gives me a QProcess::CrashExit signal in return. Not very proper.