Fixed control glitch

-Walking now handled by a function in Jack.cpp
-Previously walking didn't function naturally, because when you released
one of the movement keys, it stopped the player, regardless of if
another movement key is pressed.
-Now the player will move if and only if one key of the left or right
keys is down.

-Still working on other issues. Discovered why platforms were not
showing. It is now noted in the platform.cpp
-Most should be fixed when the camera, screen, and background functions
are made. Most likely these will be members of the Window class.