Consider how to make container "hug" content for any minor alignment

[bridger]

When a view's minor alignment is SLAlignmentFill then all subviews will fit the container and any of them that have a hugging priority will cause the container to be as small as possible. When any other alignment is specified then the container is free to get as big as it wants along the minor axis.

It would be nice to set a "hug" priority at which the minor axis will try to get as small as possible. This would allow a set of horizontal views to be top-aligned, but the container will only be as tall as the tallest subview.