More stable backend

There's not much to describe but the new approach to run the backend. Now it will be a daemon (or Launch Agent in the mac terminology) that will run when your user logs in.

To simplify the installation of the daemon, but giving you control about the process, a script is provided to install the daemon. It's easy: when installed, go to Playbox.widget and you'll see the script install_service.command. If you open it, a terminal will show and the script will install the daemon. Should go fine. And now, you have the backend properly installed.

Have fun :)