Add toggle_focus_layout to core commands?

[urob]

As a follow-up to #759, do we want to add a toggle_focus_layout command to core? This wouldn't be a priority as one can easily implement it in the csx with something like this:

    context.CommandManager.Add(
            identifier: "toggle_focus_layout",
            title: "Toggle focus layout",
            callback: () =>
            {
                IWorkspace workspace = context.WorkspaceManager.ActiveWorkspace;
                if (workspace.ActiveLayoutEngine.Name == "Focus")
                {
                    workspace.LastActiveLayoutEngine();
                }
                else
                {
                    workspace.SetLayoutEngineFromName("Focus");
                }
            }
    );

On the other hand, this seems like something others might want to use and might be worth having in the API?