Add Extended Version info to XamMac

Having an easy way to access the SDK's is convenient since this information is retrieved by the IDE.

Xamarin.MacDev/ExtendedVersion.cs

@@ -33,7 +33,7 @@ namespace Xamarin.MacDev {
 				case "Branch":
 					rv.Branch = value;
 					break;
-				case "BuildDate":
+				case "Build date":
 					rv.BuildDate = value;
 					break;
 				}

Xamarin.MacDev/XamMacSdk.cs

@@ -27,6 +27,7 @@ namespace Xamarin.MacDev {
 
 		string monoMacAppLauncherPath;
 		PDictionary versions;
+		ExtendedVersion extended_version;
 
 		public bool IsInstalled { get; private set; }
 
@@ -303,5 +304,17 @@ namespace Xamarin.MacDev {
 		public bool SupportsSiriIntents {
 			get { return CheckSupportsFeature ("siri-intents"); }
 		}
+
+		public ExtendedVersion ExtendedVersion
+		{
+			get
+			{
+				if (extended_version == null)
+				{
+					extended_version = ExtendedVersion.Read(Path.Combine(FrameworkDirectory, "buildinfo"));
+				}
+				return extended_version;
+			}
+		}
 	}
 }