Added an ADDED_TO_STAGE listener in constructor

The stage variable should not be referenced before checking if the game
has been added to the stage yet.
Changed main menu buttons to use MainButton class

Adresses Fenoxo/sourceTiTS#164
Revert "Changed main menu buttons to use MainButton class"

This reverts commit cc6fde2a673c0ab93f7f384e1ad3a56a9e0d115c.
Added func and arg properties to MainMenuButton

To allow people to tell what these buttons are doing.
Merge pull request #144 from bleachisback/master

Added an ADDED_TO_STAGE listener in constructor