fix constructors

Pilz.UI.Gtk/Dialogs/GtkContent.cs

@@ -10,6 +10,14 @@ public class GtkContent : Box
     public bool RegisterDialogAccept { get; set; } = true;
     public bool RegisterDialogCancel { get; set; } = true;
 
+    protected GtkContent()
+    {
+    }
+
+    protected GtkContent(nint raw) : base(raw)
+    {
+    }
+
     protected void Close(ResponseType result)
     {
         Result = result;