[mediaqueries] overflow-block / overflow-inline shorthand

Been thinking about the discussion yesterday, and I think I agree with
leaving the spec as-is for overflow-block / overflow-inline. But it
might also be nice to have a shorthand, which would work as follows:

     (overflow: none)   = (overflow-block: none and (overflow-inline: none)
     (overflow: page)   = (overflow-block: page)
     (overflow: scroll) = ((overflow-block: scroll) or (overflow-block: scroll-page))
                          and (overflow-inline: scroll)

[ Alternatively,
     (overflow: scroll) = (overflow-block: scroll) or (overflow-inline: scroll)
   but I think detecting scrollability at all is more useful. ]

~fantasai