More aggressive CPU usage caps

[Artoria2e5]

After #34/#32 is done, it should be possible to allow users to provide a higher CPU usage without much impact. Perhaps an option for 75% or even 100% should be added. Or even better, turn the usage setting into a 10%–100% range slider <input> with snapping (listening to change and setting the value?) to current predefined levels (10, 20, 50) for value ± 5%.

[Artoria2e5]

Just uncommenting the old <label for="max-usage"> input already gives some interestingly usable combination as the three checkboxes do sync with that range input… Still it's redundant.

Tried some snapping in update(). Feels a bit strange to see values magically change without knowing the levels.

const impactLevels = [10, 20, 50];
// ...
let snap = function snap (value, levels, threshold) {
    for (var level of levels)
        if (value >= level - threshold && value < level + threshold)
            return level;

    return value;
}
// ...
update() {
    this.maxUsage = snap(this.maxUsage, impactLevels, 5);