Drop SlotTable.scan, add composable selection support

[zach-klippenstein]

Another solution to this problem, which would be much more useful for devs, would be the ability to start the scan at some point further down in the tree, similar to passing a non-root View to scan(). However that presents a few (solvable) issues:

1. Composables don't have any implicit identity, so we'd probably need to do something like for filtering and use test tags or layout IDs to identify the root.
2. The visitor infra doesn't currently support filtering nodes above, only below, so that would have to be implemented. This could also make it possible to visit Groups directly instead of using the intermediate LayoutInfo type - I'm not sure we should, since that type lets us decouple a bit from compose APIs that might change a lot, but we'd have the option.

Having thought about this more, that API makes much more sense than this one. I'll take a stab at implementing it, but would like to not block this PR on it since this API is marked as unstable anyway.

Originally posted by @zach-klippenstein in #33 (comment)