Add uncollapse and track collapsed state

This tracks whether the field was last collapsed or uncollapsed, and
currently just kind of ignores that. It also offers the uncollapse
method to appropriately uncollapse the field. Adding proper collapsing
behavior in reloadData becomes trivial when that’s ready.
Add uncollapse and reload buttons to the example

This lets us test that uncollapse works appropriately, by providing a
basic UI on top of the features.
Add tests for uncollapse