Two independent filters. One allows only styles that are enabled, the
other allows only styles that were written locally (that is, styles
without an update URL).
(1) An element's `change` listener can be used to initialize what it's
controlling. `loadPrefs` sends a `change` event to the element
after setting the element `value` but before adding its own `change`
listener. Add the element's listener before calling `loadPrefs` to receive
the synthetic event.
(2) `prefs.setPref` only broadcasts a notification if the value returned
by `getPref` changes.
(3) A user preference with the same (typed) value as the default is
evicted from `localStorage`. (Firefox does this.)
(1) `loadPrefs` was called with string instead of boolean
(2) Sense of boolean `popup.breadcrumbs.usePath` was inverted
(3) `loadPrefs` wasn't passing default to `getPrefs`
(4) `JSON.stringify(string)` added quotes to string
(5) Preferences `manage.enabledFirst`, `observer.observeFrameContent`, `observer.observeFrameLoad` aren't used
