webutil.js: add localStorage/chrome.storage settings.

webutil.js: add localStorage/chrome.storage settings.

Add routines to store/read settings in either localStorage or in
chrome.storage.sync (which is synchronized between browsers for
extensions/apps).

Before using chrome.storage.sync the initSettings routine must to
called setup the intermediate cache which speeds up access and allows
multiple setting changes to be coallesced to avoid hitting storage
change frequency limits/quotas.