Styles are listed before actions by default. Actions are listed before
styles if `popup.stylesFirst` is false. A control for the option is shown
in "Manage" if `popup.stylesFirstUI` is true.
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) `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
