RelativeLayout + Widget + Extensions

[edgarfgp]

This PR adds basic support to match v1 for RelativeLayout based on : https://docs.microsoft.com/en-us/dotnet/api/xamarin.forms.relativelayout?view=xamarin-forms

Not Implemented

• RelativeToView as we need a way to eg the a reference to the view that we want base the position of our element

  public static Constraint RelativeToView(View view, Func<RelativeLayout, View, double> measure)
  {
  var result = new Constraint { _measureFunc = layout => measure(layout, view), RelativeTo = new[] { view } };
  
  return result;
  }

• BoundsConstraint

  
  public static BoundsConstraint FromExpression(Expression<Func<Rectangle>> expression, IEnumerable<View> parents = null)
  {
  return FromExpression(expression, false, parents);
  }

internal static BoundsConstraint FromExpression(Expression<Func> expression, bool fromExpression, IEnumerable parents = null) { Func compiled = expression.Compile(); var result = new BoundsConstraint { _measureFunc = compiled, RelativeTo = parents ?? ExpressionSearch.Default.FindObjects(expression).ToArray(), // make sure we have our own copy CreatedFromExpression = fromExpression };

return result;

}

### We could consider using ViewRef to try to accomplish this maybe ? 

[TimLariviere]

For now, let's merge this PR. Given the non-trivial support of RelativeToView and BoundsConstraint, and the fact it was never supported by v1 and no one complained about it, let's keep it for when the need arise.