Rename tree pngs to follow a logical convention.
Implement a Tree class

A random Tree sprite is picked from the data directory and used for the
tree's image. Trees absorb arrows.
Resizeable game window.
General collision detection

Actors now collide with each other when moving. Arrows are not treated as
collideable objects because they do not impede movement. This means trees
actually block movement and can be used for cover.