no `default_environment` && `new_default_environment` in SCTK

[Jedsek]

They were inroduced in book, but disappeared in now version.

[tsdh]

Yes, it seems the Environment struct with all the stuff discussed in the book were only part up to but not including smithay-client-toolkit v0.17.0 and then a big refactoring was done and a lot of what's discussed in the book vanished.

It would be very good if the book published at https://smithay.github.io/book/intro.html had a disclaimer that it corresponds to the now outdated version 0.16.0 (or whatever the right version is) and will hopefully be updated in the future (volunteers welcome!).