Tree Widget: Add API release tags

[grigasp]

At the moment the only difference between public and internal APIs is the barrel export. It's very inconvenient as it makes it hard to know whether a file is exported through the barrel (it may be exported indirectly) and error prone as it's very easy to mistakenly make a function public by adding an internal function to an exported file.

At the very least we should tag our functions. Using API exporter would help us identify possible issues (e.g. public function returning internal type) and track breaking changes in public API.