Tidy is a simple library containing some useful actors and interfaces which can be used by applications developers; it aims to be simple and yet provide some high-level classes that Clutter won’t provide.
it is by no mean complete, or aiming to replace other toolkits; you can think of it as a reference implementation for a toolkit based on Clutter.
Tidy works as a standalone toolkit, but it can also be used as a copy-and-paste repository, like libegg for the gtk+ stack; because of this, it doesn’t provide any kind of API or ABI guarantee, and it probably won’t be released in form of tarballs. it can be seen as a constant work in progress.