Unmounting nested widgets doesn't call unmount() on nested children

[ecton]

When debugging some oddities of reusing widgets in cushy-show, I realized that nested widgets never receive their unmounted events.

I tried quickly solving it, but it's a little tricker than it appears at first glance. When we go through to clean up each node, we gather our list of nodes to remove, but at this point in the loop, we can't invoke the unmounted events, because we already have the tree locked.

We're going to need to refactor widget unmounting to happen in phases so that the tree lock can be held and released. I think we need to make sure unmounted is called on every node we're about to unmount before we begin altering the tree.