Add a default button to TaskDialog if no buttons were setup

[augustoproiete]

^{Spin-off of #48}

If you attempt to call ShowDialog on a TaskDialog that you haven't added any buttons to, the call to ShowDialog will throw an InvalidOperationException.

Example:

var dialog = new Ookii.Dialogs.Wpf.TaskDialog
{
    Content = "Message",
    MainInstruction = "Instruction",
};

dialog.ShowDialog(); // InvalidOperationException

Let's add a default OK button to the TaskDialog if no buttons were setup.