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;

Pilz.UI.Gtk/Dialogs/GtkDialogBase.cs

@@ -6,7 +6,7 @@ public partial class GtkDialogBase : Dialog
 {
     public GtkDialogBase() : this(new Builder("GtkDialogBase.glade")) { }
 
-    private GtkDialogBase(Builder builder) : base(builder.GetRawOwnedObject("GtkDialogBase"))
+    protected GtkDialogBase(Builder builder) : base(builder.GetRawOwnedObject("GtkDialogBase"))
     {
         builder.Autoconnect(this);
         DefaultResponse = ResponseType.Cancel;