popup at boot

[boyska]

For a user, it is not immediately clear that the "cdrom" icon should be clicked on to create persistence. Actually, this is rather confusing for anyone.

We could do something to help them! I will put there some proposals:

1. Add a --popup-if-notfound which will be then used in the xinitrc script. This will open the window for first-time users.
2. Launch a libnotify message explaining things to the user. Clicking on the bubble should open the window.

I think that the first is better, and addresses more precisely the persistence-orientedness of freepto.