Stack in terms of relative actions

This rewrite is based on a TODO comment for the *stack_refresh*
function of *awesome*:
    It might be worth stopping to restack everyone and only stack `c'
    relatively to the first matching in the list.

And on the concept of relative stacking (via XDG_CONFIG_WINDOW_SIBLING).

Additionally the `adaptative_raise` setting was removed because it
became obsolete when the choice was made of not raising windows when
focusing via `focus_follows_pointer`.

Windows of type *desktop* are now supported (but not managed).