This sets the CurrentObject to be the result of a call to the view's GoView.PickObject to pick the selectable document object at the current point. If an object is found, what happens to the selection depends on any modifiers to the event: if GoInputEventArgs.Control is true, we toggle the selectedness of the current object; if GoInputEventArgs.Shift is true, we add the current object to the selection; otherwise we just make the current object the only selection. If no object is found and neither GoInputEventArgs.Control nor GoInputEventArgs.Shift are true, we empty the selection.