Adding item system

The game now has a rudimentary item system.  So the player now has an inventory the can carry around.  I've also created some simple menus for consuming items and buying things in shops. Godot's UI system is not as easy to use as other APIs I've used, so some of the interaction is still a bit rough.

I've also create a new tileset for interiors.  So now it's possible to enter the houses in Everion!