Update SourceView to return SourceView.Buffer

Add syntax highlighting to sample application.

Source/Libs/SourceView/SourceView.cs

@@ -0,0 +1,12 @@
+namespace GtkSource
+{
+    public partial class SourceView : Gtk.TextView
+    {
+        new public GtkSource.Buffer Buffer
+        {
+            get => base.Buffer as GtkSource.Buffer;
+            set => base.Buffer = value;
+        }
+    }
+}
+

Source/Samples/MainWindow.cs

@@ -59,6 +59,7 @@ namespace Samples
 
             _textViewCode = new SourceView();
             _textViewCode.ShowLineNumbers = true;
+	    _textViewCode.Buffer.Language = new LanguageManager().GetLanguage("c-sharp");
 
             _textViewCode.Margin = 3;
             scroll2.Child = _textViewCode;