Cleanup styling code.

src/Avalonia.Styling/Diagnostics/StyleDiagnostics.cs

@@ -0,0 +1,21 @@
+using System.Collections.Generic;
+using Avalonia.Styling;
+
+namespace Avalonia.Diagnostics
+{
+    /// <summary>
+    /// Contains information about style related diagnostics of a control.
+    /// </summary>
+    public class StyleDiagnostics
+    {
+        /// <summary>
+        /// Currently applied styles.
+        /// </summary>
+        public IReadOnlyList<IStyleInstance> AppliedStyles { get; }
+
+        public StyleDiagnostics(IReadOnlyList<IStyleInstance> appliedStyles)
+        {
+            AppliedStyles = appliedStyles;
+        }
+    }
+}

src/Avalonia.Styling/Diagnostics/StyledElementExtensions.cs

@@ -0,0 +1,17 @@
+namespace Avalonia.Diagnostics
+{
+    /// <summary>
+    /// Defines diagnostic extensions on <see cref="StyledElement"/>s.
+    /// </summary>
+    public static class StyledElementExtensions
+    {
+        /// <summary>
+        /// Gets a style diagnostics for a <see cref="StyledElement"/>.
+        /// </summary>
+        /// <param name="styledElement">The element.</param>
+        public static StyleDiagnostics GetStyleDiagnostics(this StyledElement styledElement)
+        {
+            return styledElement.GetStyleDiagnosticsInternal();
+        }
+    }
+}

src/Avalonia.Styling/StyledElement.cs

@@ -16,24 +16,6 @@ using Avalonia.Styling;
 
 namespace Avalonia
 {
-    public class StyleDiagnostics
-    {
-        public IReadOnlyList<IStyleInstance> AppliedStyles { get; }
-
-        public StyleDiagnostics(IReadOnlyList<IStyleInstance> appliedStyles)
-        {
-            AppliedStyles = appliedStyles;
-        }
-    }
-
-    public static class StyledElementExtensions
-    {
-        public static StyleDiagnostics GetStyleDiagnostics(this StyledElement styledElement)
-        {
-            return styledElement.GetStyleDiagnosticsInternal();
-        }
-    }
-
     /// <summary>
     /// Extends an <see cref="Animatable"/> with the following features:
     /// 

src/Avalonia.Styling/Styling/IStyleInstance.cs

@@ -14,6 +14,9 @@ namespace Avalonia.Styling
         /// </summary>
         IStyle Source { get; }
 
+        /// <summary>
+        /// Gets a value indicating whether this style is active.
+        /// </summary>
         bool IsActive { get; }
 
         /// <summary>