Thank you for your suggestion, but unfortunately this is not a straight-forward task.
If we add such properties there are a lot of questions to answered in order to implement them. First of all we need to know relative to whom should be this position. Another problem is when we need to update the Left and Right properties using the relative values. For example - in your case you need to have relative coordinates from a button - imagine that this button moves - because of the layout or because of a RenderTransform. It is very complicated to determine when we should update the position.
Because of these problems we proffered to the user of the RadWindow control to implement this in the manner he/she prefers to do that.
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