Adding simple prototype of Menu

This pull request uses a provided "button" entity which can currently be spawned by pressing "ENTER". This is only for testing purposes. You can click the buttons using the RIGHT CLICK of your mouse. In the near future, we could simply click the button, then execute a function such as "endTurn( )" and then kill the button. Ideally, we can spawn a button under the following situations:

* After a player entity has moved, we can spawn two buttons, "Confirm" and "Go back". For these two to work, we need to be able to click a unit and then click a square for the unit to move to automatically. After the movement is done, we can spawn the buttons and allow the [human] player to choose to confirm their choice, or click "Go Back" to go back to the starting position. If A* saves the path, this would be great so that we can teleport the player back to their starting position.

* If the [human] player clicks a tile with the mouse that does not collide with an entity, we can spawn a button which allows them to end the turn prematurely. 

* Currently, I have no idea how a trade system GUI would work. Even though we are not ready for this, I feel that it will need a different type of GUI to handle it. For now, I feel that we should handle the simple cases with buttons.