Adds Content, ContentStatus and ContentType implementations U4-922 and U4-923.

Hiding language property untill its fully implemented.
Making set on Parent Id private.
Resolves a naming conflict in DocumentExtensions and DefaultPublishedMediaStore.

src/Umbraco.Core/Models/ContentStatus.cs

@@ -0,0 +1,10 @@
+namespace Umbraco.Core.Models
+{
+    /// <summary>
+    /// Enum for the various statuses a Content object can have
+    /// </summary>
+    public enum ContentStatus
+    {
+        Unpublished, Published, Expired, Trashed, AwaitingRelease
+    }
+}