XWiki.widgets.Suggest widget should work in a positioned container

While the Suggest widget allows specifying a parent node where the suggestions container must be placed, the position of that container is always computed as the cummulativeOffset from the affected input to the document body, even though the parent node (or one of its ancestors) could set a new position offset container by specifying a value for the CSS 'position' property other than 'static'.

This makes it almost impossible to correctly position a suggest widget attached to an input inside a modal popup.