[Design] Allow numeric values for FontSize (#2224)

This ensures that intellisense in Visual Studio
does not report an error when numeric values are
put in this property instead of the 'string' values
like 'Small', 'Large' etc

Xamarin.Forms.Core.Design/AttributeTableBuilder.cs

@@ -32,7 +32,7 @@ namespace Xamarin.Forms.Core.Design
 			foreach (var fontElement in fontElements) {
 				AddCallback (fontElement, builder => builder.AddCustomAttributes (
 					"FontSize",
-					new System.ComponentModel.TypeConverterAttribute (typeof (EnumConverter<NamedSize>))));
+					new System.ComponentModel.TypeConverterAttribute (typeof (NonExclusiveEnumConverter<NamedSize>))));
 			}
 
 			// TODO: OnPlatform/OnIdiom

Xamarin.Forms.Core.Design/NonExclusiveEnumConverter.cs

@@ -0,0 +1,12 @@
+using System.ComponentModel;
+
+namespace Xamarin.Forms.Core.Design
+{
+	internal class NonExclusiveEnumConverter<T> : EnumConverter<T>
+	{
+		public override bool GetStandardValuesExclusive(ITypeDescriptorContext context)
+		{
+			return false;
+		}
+	}
+}

Xamarin.Forms.Core.Design/Xamarin.Forms.Core.Design.csproj

@@ -51,6 +51,7 @@
   <ItemGroup Condition=" '$(OS)' != 'Unix' ">
     <Compile Include="AttributeTableBuilder.cs" />
     <Compile Include="EnumConverter.cs" />
+    <Compile Include="NonExclusiveEnumConverter.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="RegisterMetadata.cs" />
   </ItemGroup>