Overview: constrain maximum width with HdyClamp (updated)

This supersedes !416 (closed), updated to incorporate review comments and call hdy_init() correctly for libhandy-1, and submitted as a new MR because I can’t push to the old one (@jsparber didn’t allow pushes from other g-s developers to the MR; the default value for that checkbox in the MR options in GitLab is wrong).

Fixes: #890 (closed)