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/objects/Slider.java

@@ -187,9 +187,8 @@ public class Slider implements GameObject {
 
 		// start circle
 		hitCircle.drawCentered(x, y, color);
-		if (!overlayAboveNumber) {
+		if (!overlayAboveNumber)
 			hitCircleOverlay.drawCentered(x, y, Utils.COLOR_WHITE_FADE);
-		}
 
 		// ticks
 		if (ticksT != null) {