Gweather 4.0 tracking

A list of things that need to happen before GNOME 42, in order to release Gweather 4.0:

use G_DECLARE_* and G_DEFINE_* macros
switch inclusion guard to #pragma once
use explicit symbol visibility and drop the symbols map
use C99
use SPDX license identifiers and copyright text tags instead of free form licensing blurbs
ensure that the namespace version matches the version of the library
split GTK3 widgets into a separate shared library and/or delete them entirely
move tests under their own location, and run them
add more tests
document all public entry points
use a consistent coding style that can be machine generated, and run the coding style check under the CI pipeline
run a static analysis tool
remove a bunch of old API (!123 (merged))
~~use GListModel across core data types~~
ensure that the API is fully introspectable
replace hand-written refcounting with proper GLib refcounting or switch to full object types
port core applications to the new API before the freeze