port to GTK4

This is a port to GTK4 and libadwaita. It includes a rewritten graph view with guiding lines instead of polynomial interpolation. It also makes use of libadwaita widgetry and style classes where no libhandy equivalent was used, but stays close to Usage as it was before otherwise and does not contain a major design overhaul.

fixes #100 (closed), #108 (closed), closes #40 (closed), #53 (closed) as obsolete

depends on and contains !100 (merged), !102 (merged), !110 (merged), !112 (merged), !116 (merged)