Update FindDeepestView to support Adornment

[tig]

• There are no dedicated unit tests for FindDeepestView right now. We need some. Write some that test the current v2_develop version that prove it works.

• The current version ignores Adornments and returns a view if x/y are anywhere within view.Frame.

• Simplify FindDeepestView to NOT have the out params.

• Modify FindDeepestView to return Margin, Padding, or Border if x/y match. In Application.OnMouseEvent, temporarily ignore these. Eg. like this:

        var view = View.FindDeepestView (Current, a.MouseEvent.X, a.MouseEvent.Y, out int screenX, out int screenY);

        // TODO: Remove this temporary filter:
        if (view is Adornment adornment)
        {
            view = adornment.Parent;
        }

• Add FindDeepestView unit tests to prove this change works.

• Delete all the Adornment-specific code in Application.OnMouseEvent and add MouseEvent handling code to Adornment, Margin, Border, and Padding as appropriate (e.g. Adornment.MouseClick += Parent.SetFocus()).

• Don't worry about Adornment.SubViews yet. That should happen in either THIS PR or the Close Box PR.