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

See also: !120 (merged)