Added constants for Application (section aliases), System (node IDs) and MediaType aliases.

src/Umbraco.Core/Constants-System.cs

@@ -0,0 +1,26 @@
+namespace Umbraco.Core
+{
+	public static partial class Constants
+	{
+		/// <summary>
+		/// Defines the identifiers for Umbraco system nodes.
+		/// </summary>
+		public static class System
+		{
+			/// <summary>
+			/// The integer identifier for global system root node.
+			/// </summary>
+			public const int Root = -1;
+
+			/// <summary>
+			/// The integer identifier for content's recycle bin.
+			/// </summary>
+			public const int RecycleBinContent = -20;
+
+			/// <summary>
+			/// The integer identifier for media's recycle bin.
+			/// </summary>
+			public const int RecycleBinMedia = -21;
+		}
+	}
+}