prepare subtree split for a standalone library

This replaces #118 to keep everything in the bundle repo, but moving all files meant to be available for the library into a dedicated folder (which can then be used for a subtree split).

It still is a heavy BC break caused by lots of namespace changes, so I'm not sure if it's worth it. I even tried replacing classes within the old namespaces by proxies, but that [doesn't work](https://travis-ci.org/craue/CraueFormFlowBundle/builds/28439951) for all of them.