Prevent multiple tray app instances from running

[drittich]

This is a fix for issue #90.

[drittich]

It might be preferable to show the message as Information instead of a Warning. Let me know, i.e.:

vs.

[anthonylavado]

Yeah, I think Info would be nicer in this case, as there isn't a problem.

Also don't forget a period at the end of the notice message.

thank you for this!

[drittich]

Done and pushed