Cleaning up + preliminary cubes

-Deleted redundant SpikeRow class from previous game
-Fixed problems with circular inclusion
-Fixed by use of forward declaration in Character and Entity classes
-Cleaned up Entity::ApplyVelocity code
-Made a new function called MoveBackIfColliding()
-Position and Velocity variables are passed to it by reference
-Test cubes added to Level class
-They do very little at the moment, and are represented by the
decapitated head of the player
-The player now a member of the Level class
-Movement of all objects is now handled by Level::MoveObjects
-This contains old player movement code which used to be in the main
-The main is SO TINY now
-New Music considerations.txt file
-List of music I would like to implement for the different dimensions if
possible
-Not important at all right now