Level - Added gate-collision-detection, along with a way to level-up, which resets the current level for now.
Refactor - Added "this.game" throughout ArcadeGame; refactored ArcadeGame to have a "setup" phase that's different from the "startGame" and "startLevel" phases.
Level - Added "currentLevel", and move the player back to (0, 0) when level starts
Mouse Input - Lowered sensitivity from 3 to 1.  Fixed bug with windowSize; it's now based off the element's size, not the window's dimensions.