Edited debug mode, new shape-class, some more

* New debug mode has three states: 0, 1 and 2. In 0, there is equivalent
to previous "False", 1 is the same as previous "True" and 2 is a new
state that shows all objects' AABB:s
* The new shape class is Box, intended to replace Cube entirely since a
cube is just a special case of a box.
* Beginning to new class, Interactive_object. It is a base class for
objects such as buttons, levers etc. that the player interacts with
in-game.
* Some other minor changes