Create a generic floating toolbar aligner

[matthew-carroll]

Currently we have a Cupertino toolbar aligner. This alignment strategy is reasonable for other toolbars, too, but it's confusing to use a Cupertino aligner in non-Cupertino use-cases. Create a generic floating toolbar aligner. If the generic aligner works the same way as Cupertino, consider making the Cupertino aligner a dumb delegate to the generic strategy. In theory we could remove the Cupertino aligner, if the generic aligner is the same, but we don't want to break all the uses of the Cupertino aligner.