major improvements to "scene" class

Engine Changes:
-added more methods to "scene" class
-scene class now loads/saves more data
-added foreground, middleground, and background to "scene" class

Demo Changes:
-added Starcraft 2 background, portal middleground, and angrybirds
foreground
-added checkboxes to show/hide foreground,middleground, and background
-added checkbox to show/hide textures
-added checkbox to mute/unmute all sounds