Add Toplevel.Ready event

[tig]

This PR addresses a common problem found in building Terminal.Gui apps...namely, I frequently need to do some setup/initialization code AFTER Application.Run is called. This is often things having to do with setting focus or changing layout, which is all first computed as Application.Run executes.

#445 explains the rationale more fully.

The challenge is addressed by introducing a Ready event to Toplevel:

		/// <summary>
		/// Fired once the Toplevel's <see cref="T:Terminal.Gui.MainLoop"/> has started it's first iteration. 
		/// Subscribe to this event to perform tasks when the <see cref="T:Terminal.Gui.Toplevel"/> has been laid out and focus has been set.
		/// changes. A Ready event handler is a good place to finalize initialization after calling `<see cref="T:Terminal.Gui.Application.Run"/>(topLevel)`. 
		/// </summary>
		public event EventHandler Ready;

Applications then can then implement an event handling method for Ready where they can do things like set which control gets focus. E.g.:

_top.Ready += (o, e) => _top.SetFocus (_rightPane);
Application.Run(_top);

The latest UI Config PR branch (#387) utilizes Toplevel.Ready to ensure the right-hand pane retains focus after a Scenario is run.