Adwaita: Use a mixin for visible focus rings

Adwaita: Use a mixin for visible focus rings

The focus-ring() mixin allows we to flexibly style the focus ring for
each widget. By using this, we can get rid of the "Outlines" section,
which is out of place in the _common.scss file.

This commit also has the following changes:

- Transition the focus rings on most widgets.
- Add a missing focus ring to iconview.
- Move the expander-widget focus ring to its title.
- Move the notebook focus ring to its checked tab.

Closes https://gitlab.gnome.org/GNOME/gtk/-/issues/2653