The new major version of the toolkit is a requirement to fix old issues and enable future enhancements.

Update symbols and adapt logic to API changes.
Update and simplify UI definitions.
Update local copy of places sidebar and places view.
Replace dependencies with their GTK4-compatible successors.
Make a minimum changes required to build and run, with known
regressions to be fixed in future commits.

For a detailed breakup of the changes, see the 36 commits-deep
log leading to d5763fac.

This is the main part of #276