Added freeware icons for future use in prefs dialog
from http://www.iconarchive.com/show/developpers-icons-by-sekkyumu/Add-Green-Button-icon.html
aka http://sekkyumu.deviantart.com/art/Developpers-Icons-63052312
Changed prefs dialog tabs so remembers prev selected tab when re-open; added icons

Prefs now uses prefpanes instead of tabbox and tabs.  Allows
Firefox to automatically return to the last-shown tab when re-opening.

Added icons to the tabs so they look more like tabs.