Filter a tree

[kellerza]

Would you be interested to add filtering to a tree?

Example of filter behavior here

This PR moves flattenedItem and flattenItems to utils and adds flattenFilter to achieve this

flattenFilter needs a filter function and a list of 'visible' branches.
Output the flattenedItems & expanded keys (maybe this could be useful in the future, but no need to do anything with this)
Once filtered, it would be great to expand a certain branch (adding the key to forceVisible achieves this)

I would appreciate some guidance on the API