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. 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.