Add a forceLayout function to positioners

Positioners do not do actual positioning until polish. This can sometime cause issues in imperative code. For example, changing the visibility of the child of a hidden Row will not update the Row's geometry until it is shown.

The View elements contain a forceLayout function (added in 53e317468626284fe26877659dff551ce3bc0e55) that would be useful in cases like this – consider making such as function available to the Positioner elements as well.