Move Sound Tray code from FlxGame

In `FlxGame`, rather than create a new Sound Tray instance and add it to the stage, all the logic for drawing the sound tray is spread across various functions in the class.

I feel the logic should be moved out and separated into a SoundTray class. I still need to look deeper into the code and see whether or not it should be considered a plugin, or if it needs to be in the main package.
