Added tabs counter

Now with useless resource string (why not to remove descriptions, tag names are already self-describing) and variants for SeaMonkey and Thunderbird
@KOLANICH please add this line to all `variables.properties` files in the repository (`extension/chrome/locale/*/variables.properties`). It's OK to add  as unchanged i.e. untranslated strings.

The guys at BabelZilla will do the rest! :)