[Anton] Started work on singleplayer logic.

I've added a warning popup which triggers if you have not filled all the
ship slots, which prevents you from saving an incomplete ship. Also, if
you have never created a ship before, you will automatically be taken to
the ship builder in order to create one (might add a popup later). I've
made sure that there are no ways to bypass this, so there should not be
any weird crashes later due to wrong ship files.