mainmenu music bar tweaks

Make it work like in osu:
  separate play/pause button
  stop button
  pressing play either resumes or start from beginning
  pressing pause pauses and unpauses
  pressing stop pauses and sets position at the start
Less aggressive expand scale
Icons from fontawesome, like osu
Seek bar: correct size, non-rounded corners and less white color
Positioning tweaks

src/itdelatrisu/opsu/ui/Colors.java

@@ -28,6 +28,7 @@ public class Colors {
 		BLACK_ALPHA     = new Color(0, 0, 0, 0.5f),
 		BLACK_ALPHA_75  = new Color(0, 0, 0, 0.75f),
 		BLACK_ALPHA_85  = new Color(0, 0, 0, 0.85f),
+		WHITE_ALPHA_75  = new Color(1, 1, 1, 0.75f),
 		WHITE_ALPHA     = new Color(255, 255, 255, 0.5f),
 		BLUE_DIVIDER    = new Color(49, 94, 237),
 		BLUE_BACKGROUND = new Color(74, 130, 255),