Add icon
Modified http://findicons.com/icon/115581/tab_new (http://tango.freedesktop.org/) + chrome://browser/skin/Privacy-16.png
Add "Private Tab" checkbox to tab context menu
API: dispatch "PrivateTab:PrivateChanged" event for other extensions
Simplify: tab state now updated automatically after toggleTabPrivate()
Tab context menu for SeaMonkey
+ correct destroyControls() (wrong "this" in previous version)