Reinstate StyleClass but keep @class as requested (#2044)

Xamarin.Forms.Core/VisualElement.cs

@@ -278,8 +278,7 @@ namespace Xamarin.Forms
 			set { SetValue(StyleProperty, value); }
 		}
 
-
-		[Obsolete("Use @class")]
+		
 		[TypeConverter(typeof(ListStringTypeConverter))]
 		public IList<string> StyleClass {
 			get { return @class; }

docs/Xamarin.Forms.Core/Xamarin.Forms/VisualElement.xml

@@ -1819,9 +1819,6 @@
         <AssemblyVersion>2.0.0.0</AssemblyVersion>
       </AssemblyInfo>
       <Attributes>
-        <Attribute>
-          <AttributeName>System.Obsolete("Use @class")</AttributeName>
-        </Attribute>
         <Attribute>
           <AttributeName>Xamarin.Forms.TypeConverter(typeof(Xamarin.Forms.ListStringTypeConverter))</AttributeName>
         </Attribute>