[GTK] Use gtk_file_chooser_set_filename for open dialogs

samples/ControlCatalog/Pages/DialogsPage.xaml.cs

@@ -1,4 +1,7 @@
+using System;
 using System.Collections.Generic;
+using System.Linq;
+using System.Reflection;
 using Avalonia.Controls;
 using Avalonia.Markup.Xaml;
 #pragma warning disable 4014
@@ -34,7 +37,9 @@ namespace ControlCatalog.Pages
                 new OpenFileDialog()
                 {
                     Title = "Open file",
-                    Filters = GetFilters()
+                    Filters = GetFilters(),
+                    // Almost guaranteed to exist
+                    InitialFileName = Assembly.GetEntryAssembly()?.GetModules().FirstOrDefault()?.FullyQualifiedName
                 }.ShowAsync(GetWindow());
             };
             this.FindControl<Button>("SaveFile").Click += delegate

src/Avalonia.X11/NativeDialogs/GtkNativeFileDialogs.cs

@@ -91,7 +91,13 @@ namespace Avalonia.X11.NativeDialogs
                 gtk_dialog_add_button(dlg, open, GtkResponseType.Cancel);
             if (initialFileName != null)
                 using (var fn = new Utf8Buffer(initialFileName))
-                    gtk_file_chooser_set_current_name(dlg, fn);
+                {
+                    if (action == GtkFileChooserAction.Save)
+                        gtk_file_chooser_set_current_name(dlg, fn);
+                    else
+                        gtk_file_chooser_set_filename(dlg, fn);
+                }
+
             gtk_window_present(dlg);
             return tcs.Task;
         }