Refactored the display of grid units

Added a PRESSED state
Made GridUnit._state public
Created a hash of various grid unit states to images
GridUnit.draw method determines which image to draw
Removed several GridUnit setters
Implemented toggling of grid units
Moved logic from mouse clicked event to mouse released
Added a Utility class for miscellaneous helper functions
Converted GridUnit.adjacentUnits hash values from Point to GridUnit