Add quick settings timer

Adds a new quick settings timer/stopwatch module with timer and chrono modes, preset controls, and finish notifications. Updates the README with usage details and screenshots, and adds the new icon/image assets.

images/plus.svg

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+<rect x="2" y="2" width="44" height="44" rx="8" fill="white"/>
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+</svg>

images/plus_circle.svg

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+<svg width="48" height="48" viewBox="0 0 48 48" fill="none" xmlns="http://www.w3.org/2000/svg">
+<rect x="2" y="2" width="44" height="44" rx="8" fill="white"/>
+<path d="M15 22.5H19.5C21.1569 22.5 22.5 21.1569 22.5 19.5V15H25.5V19.5C25.5 21.1569 26.8431 22.5 28.5 22.5L33 22.5V25.5L28.4932 25.5C26.8395 25.5037 25.5 26.8454 25.5 28.5V33H22.5V28.5C22.5 26.8454 21.1605 25.5037 19.5068 25.5L15 25.5V22.5Z" fill="black"/>
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+</svg>