* Add a Level Exit doodad, which for now is a little blue flag on a pole
that reads "END"
* JavaScript API: global function EndLevel() will end the level. The
exit doodad calls this when touched by the player.
* Add a "Level Completed" alert box UI to PlayScene with dynamic button
layouts.
* The alert box pops up when a doodad calls EndLevel() and contains
action buttons what to do next.
* "Play Again" restarts the current level again.
* "Edit Level" if you came from the EditorScene; otherwise this button
is not visible.
* "Next Level" is a to-be-implemented button to advance in the single
player story mode. Only shows up when PlayScene.HasNext=true.
* "Exit to Menu" is always visible and closes out to the MainScene.
