Port to GTK4

Mocks: https://gitlab.gnome.org/Teams/Design/app-mockups/-/blob/master/weather/weather.png

Dependency updates

• Drop Libhandy for Libadwaita v1
• Port to libgweather 4 courtesy of @ebassi's work
• GNOME Desktop is dropped because we no longer use the clock widget
• We still set the org.gnome.GWeather gsettings for backwards compatibility but we read (and write) from org.gnome.GWeather4

Changes

• Port the LocationEntry code to Weather (libgweather dropped it)
  • Port Geolocation code
  • Separate model management for selected and searching
    • Move string filtering into JS for performance
    • Avoid recursively populating a model with the entire World and 4 strings per location
      • Strings are cached and we only select cities and weather stations
• Port to ESM
• Port thermometer view to render nodes
• Fix style regressions
• Support dark mode
• Adapt to GTK4 a11y APIs
• Move more UI code into templating