this variant is known to work, but focus() is ugly - we need in general a better way to handle multihead, this issel-stuff looks awkward (maybe it might be a good idea to set sel to NULL but to introduce a Client *revert which is set if a screen is unfocused, have to think about it further).

applied a modified version of Christof Musik's multihead patch (though this is not sure if it works in all cases, have to wait for an ACK by Christof)