Made some class structure changes

Added a new class GameMode as a superclass to SpaceMode and the future
PlatformerMode class. Minor changes were made to Element and its derived
classes. The change caused a linking error in the VS project.

TL;DR:    IT'S BROKEN
Added null pointer exception
Implemented despawn hack

Added a new variable to Entity called "doDraw" which can be set to false
to prevent an Entity from being drawn.