Allow Rulesets to create a non-FontAwesome icon

This also
- Renames TextAwesome to SpriteIcon.
- Removes the default size of "20" from SpriteIcon (now defaults to the underlying texture size).

osu.Game.Rulesets.Taiko/Objects/Drawables/Pieces/SwellSymbolPiece.cs

@@ -3,20 +3,20 @@
 
 using osu.Framework.Graphics;
 using osu.Game.Graphics;
+using OpenTK;
 
 namespace osu.Game.Rulesets.Taiko.Objects.Drawables.Pieces
 {
     /// <summary>
     /// The symbol used for swell pieces.
     /// </summary>
-    public class SwellSymbolPiece : TextAwesome
+    public class SwellSymbolPiece : SpriteIcon
     {
         public SwellSymbolPiece()
         {
             Anchor = Anchor.Centre;
             Origin = Anchor.Centre;
-            UseFullGlyphHeight = true;
-            TextSize = CirclePiece.SYMBOL_INNER_SIZE;
+            Size = new Vector2(CirclePiece.SYMBOL_INNER_SIZE);
             Icon = FontAwesome.fa_asterisk;
             Shadow = false;
         }