added AssetManager, added Music Volume Slider

Adding assetmanager restructured many things:  Assets used during
thesplash screen are manually loaded at game launch; the splash screen
calls assetmanager.update() during its render; it checks if music has
been loaded and if so starts playing it immediately; once all assets are
loaded, the splash screen notifies the main application, which then
instantiates all other game screens; final the main application notifies
the splash screen that all other screens are loaded, signaling that it
may advance. The end result is minimal 'start-up time' prior to the
splash screen being displayed.