Add GetEnabled to EnumExtensions

[jbreuer]

The EnumExtensions now contains the following methods: GetKey GetLabel

The EnumDataSource attribute also has an Enabled property. It would be nice if you could also return that boolean with a GetEnabled extension method.

[mistyn8]

Looks like this existed at some point but seems Enabled removed now as no property on EditorDataItem? {nuPickers/Shared/Editor/EditorDataItem.cs}

also removed in the EnumDataSource {/nuPickers/Shared/EnumDataSource/EnumDataSource.cs}

 case "Enabled":
        //enabled = (bool)customAttributeNamedArguement.TypedValue.Value;
        break;