Re-factor `Cursor` handling per `./docfx/docs/cursor.md`

[tig]

Related:

• #3432

See:

https://github.com/gui-cs/Terminal.Gui/blob/v2_develop/docfx/docs/cursor.md

Note, in this we need to fix Focused and MostFocused such that these unit tests are addressed:

    [Fact, Trait("BUGBUG", "Fix in Issue #3444")]
    public void Focused_NoSubviews ()
    {
        var view = new View ();
        Assert.Null (view.Focused);

        view.CanFocus = true;
        view.SetFocus ();
        Assert.True (view.HasFocus);
        Assert.Null (view.Focused); // BUGBUG: Should be view
    }

    // View.MostFocused - No subviews
    [Fact, Trait ("BUGBUG", "Fix in Issue #3444")]
    public void Most_Focused_NoSubviews ()
    {
        var view = new View ();
        Assert.Null (view.Focused);

        view.CanFocus = true;
        view.SetFocus ();
        Assert.True (view.HasFocus);
        Assert.Null (view.MostFocused); // BUGBUG: Should be view
    }