Do not allow launching two times Publii + Icone( Feature )

[hubyhuby]

I often end relaunching publii by mistake there are two reasons for that :

1. On linux there is no icone in my task bar, so I don t see it
2. Due to not having hot code reloading (or preview in publie) I have many windows open and 50 tabs in chrome with "previews"

=> solution => check at publii startup if an instance is already running.