Implement SearchableTreeWidget for Metadata

[evamaxfield]

pyapp-kit/superqt#158

Better metadata exploration and navigation. Might need to convert the XML / etree metadata structs to dict but might give it a try with just passing it in.

[andy-sweet]

If anyone picks this up and struggles with that superqt addition, let me know I can try to improve it. For XML, it could make sense to add something like a fromIterable factory method and just create a tree with a single column.