Add setting and CSS media query for reduced motion

Emmanuele Bassirequested to merge
ebassi/prefer-reduced-motion into main

Disabling every animation is not an accessibility setting: animations convey meaning, and turning all of them off is actually a great way to make user interfaces less accessible.

Platforms (macOS, Android, Windows, web) have converged towards the approach of adding a setting to reduce motion inside animations, in order to avoid triggering discomfort for those with vestibular motion sensitivity, or distracting for those with attention deficits.

We implement the same mechanism through GtkSettings, for animations defined inside widgets; and via media queries, for CSS animations.

See: #6757 (closed), !7344

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