Make BufferView::singeParUpdate more robust

Make BufferView::singeParUpdate more robust

In some cases, it might happen that this method is called in cases
where no metrics is know for the current paragraph or where its
position is not set.

Take care of these cases to avoid assertions.

Remove setting of inset positions in the method, but make sure that
updateMetrics(false) is always called to get everything right.

In the new code, updateMetrics(bool) os the method that sets
everything right with minimal effort.