Follow-up to #64.

- Removed NEW_SLIDER option, and use the skin "SliderStyle" instead.  Uses the new style by default, unless STYLE_PEPPYSLIDER is specified.
- Check if OpenGL 3.0 is supported before trying to draw new style sliders.
- Fixed compilation warnings; removed unneeded fields and imports.
- Filled in some missing Javadocs.
- Style changes.

Signed-off-by: Jeffrey Han <itdelatrisu@gmail.com>

src/itdelatrisu/opsu/states/OptionsMenu.java

@@ -59,8 +59,7 @@ public class OptionsMenu extends BasicGameState {
 			GameOption.SCREENSHOT_FORMAT,
 			GameOption.NEW_CURSOR,
 			GameOption.DYNAMIC_BACKGROUND,
-			GameOption.LOAD_VERBOSE,
-			GameOption.NEW_SLIDER
+			GameOption.LOAD_VERBOSE
 		}),
 		MUSIC ("Music", new GameOption[] {
 			GameOption.MASTER_VOLUME,