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
Thanks!
Please use the same string (i.e. `Count of visible tabs`) as in `en-US` locale.
Simply just append the `en-US` line (`variable.TabsCount.description=Count of visible tabs`) to the other `variables.properties` files.


One more commit please for `sv-SE`: `extension/chrome/locale/sv-SE/variables.properties`!