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. For example:

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• Add a missing focus ring to iconview.

• Move the expander-widget focus ring to its title.

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• Move the notebook focus ring to its checked tab.

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Closes #2653 (closed)

CC @jimmac @frederik.feichtmeier