Implemented back button in AboutScene

A back button is added and implemented in the AboutScene.java class
Implemented back button in RankingsScene

A back button is added and implemented in the RankingsScene.java class
Implemented back button is BadgesScene

A back button is added and implemented in the BadgesScene.java class. Also some code is modified for the looks
Implemented back button in StartScene

A back button is added and implemented in the StartScene.java class
Removed unused code

The inner class BackButton wasn't used so it is now removed
Merge branch 'BackButton'