Closes #6, GUI slicker, Maps added

ObjectActions performed on Boat rewritten. Now have an "active field" which allows Boat to delete an ObjectAction if no more active. When removed, an ObjectAction can now perform a last action, such as setting back original max speed to a boat (ex: RockEffect)
Rock management in VersusGame also changed, and made simpler.
FireAction also changed a bit, but not in a logical way, just to follow new ObjectAction structure.
GUI made slicker by finally fixing the problem where one could see a menu "blink" in the top left corner or on the upper bound of the screen before beeing displayed centered on screen. Looks much better!
Folder with maps added so that contributers could use "Test" section in Play menu.