Map transitions and state saving

You'll be able to freely traverse back and forth between areas of the game, which will allow for some non-linear - perhaps hub - locations. To support this, I needed to make sure the state of the player, the map, and all gameobjects and items are serialised to json and saved when you exit to a new area.

This also covers general game saving. The state is stored when the game is paused or exited at system-level. When you relaunch the game, the state is loaded behind the title screen and continues where you left off!