Insert the urlbar icon at the beginning of #urlbar-icons

Firefox 23 removed the bookmark star from the urlbar, so using
the star as an insertion point reference doesn't work so well now.
Move from resource:// to chrome://

This increases the minimum Firefox requirement to 10.0, but I'm going to
bump it straight to 17.0 because that gets me for...of loop support (and
I can't be bothered to test on older versions.)
Use JS module for watchWindows()/unload()
Use Servies.jsm and Ci where possible
Bump version number and compatibility
"Add-on" to "extension" in comments

Because it's an extension.
Automatically detect NAT64 prefix