[dev] [tabbed] Add window ID to environment

Hi people,

the attached patch lets tabbed expose its window ID to its children via
the env var TABBED_WIN. This allows doing things like the following from
inside tabbed:

    st -t "Foo" -w $TABBED_WIN &
    st -t "Bar" -w $TABBED_WIN

thus easing the use of different tab titles.

-- 
    $l="\n";$p=q-sub r{rand}sub c{((shift)**2+(shift)**2)<1}while(
    $i++<=$s){$t++if c r,r}die("Gregor Best, 0xDB9F9A7C, ".($t/$s*
    4).$l)-;$s+=ord$$|$_ for split //,$p;srand($s|$$);eval$p;die$@