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/Graphics/Containers/ConstrainedIconContainer.cs

@@ -0,0 +1,46 @@
+// Copyright (c) 2007-2017 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using System;
+using osu.Framework.Graphics;
+using osu.Framework.Graphics.Containers;
+using OpenTK;
+
+namespace osu.Game.Graphics.Containers
+{
+    /// <summary>
+    /// Display an icon that is constrained to a physical region on screen.
+    /// </summary>
+    public class ConstrainedIconContainer : Container
+    {
+        public Drawable Icon
+        {
+            get
+            {
+                return InternalChild;
+            }
+
+            set
+            {
+                InternalChild = value;
+            }
+        }
+
+        protected override void Update()
+        {
+            base.Update();
+            if (InternalChildren.Count > 0 && InternalChild.DrawSize.X > 0)
+            {
+                var fitScale = Math.Min(DrawSize.X / InternalChild.DrawSize.X, DrawSize.Y / InternalChild.DrawSize.Y);
+                InternalChild.Scale = new Vector2(fitScale);
+                InternalChild.Anchor = Anchor.Centre;
+                InternalChild.Origin = Anchor.Centre;
+            }
+        }
+
+        public ConstrainedIconContainer()
+        {
+            Masking = true;
+        }
+    }
+}