* Changes the class names so that they all start with `setting-`.
That should make it harder to accidentally use a setting class outside
the settings popover, where loading the CSS might accidentally change
the styles of something unrelated.
* Get rid of an unnecessary wrapper DIV around the radio button line.
* Simplify CSS selectors by making the DOM easier and more intuitive
to target.
rustdoc: remove redundant CSS rule `#settings .setting-line`
Since the current version of settings.js always nests things below a div with ID `settings`, this rule always overrode the one above.
This prevents some strange blur-event-related bugs with the "?" command
by ensuring that the focus remains in the same spot when the settings
area closes.
