Prefer gtkmm4 to gtkmm3

This tutorial describes gtkmm-3.0. The latest ABI/API is gtkmm-4.0. Use gtkmm4 in new applications.

The reference documentation of gtkmm3 is no longer available online. The links to reference documentation in this document link to descriptions of classes with the same name in gtkmm4. There may be broken links when there is no such class in gtkmm4.