Replaced paint(getGraphics()) with repaint()
Refactored click actions and mine logic

GamePanel event listeners now pass actions to GridUnits instead of taking action on behalf of the GridUnits
Fixed and moved broken grid construction to the constructor, resetGrid clears mines
Moved exposing of mines logic to GridUnit, now recursive
Removed GridUnit.State enumeration, replaced with several boolean variables, modified GridUnit.draw method to use these
Removed some unnecessary getters, setters, and variables
Made most GridUnit fields public for convenience

Having both a free getter and setter means that they are essentially public anyway
Changed the naming scheme of GridUnit variables

For example, "checked" became "isChecked"