[TUI] Add Main Help dialog with F1 shortcut and Ok command

Aaru.Tui/ViewModels/Dialogs/MainHelpDialogViewModel.cs

@@ -0,0 +1,24 @@
+using System.Windows.Input;
+using CommunityToolkit.Mvvm.Input;
+using Iciclecreek.Avalonia.WindowManager;
+
+namespace Aaru.Tui.ViewModels.Dialogs;
+
+public sealed class MainHelpDialogViewModel : ViewModelBase
+{
+    internal ManagedWindow _dialog = null!;
+
+
+    public MainHelpDialogViewModel(ManagedWindow dialog)
+    {
+        _dialog   = dialog;
+        OkCommand = new RelayCommand(Ok);
+    }
+
+    public ICommand OkCommand { get; }
+
+    void Ok()
+    {
+        _dialog.Close(true);
+    }
+}