elm/genlist: hide cached item contents and mark content unfocusable during calc

elm/genlist: hide cached item contents and mark content unfocusable during calc

cached item contents should already be hidden by the edje clipper, so this simply
changes their visible state to break them out of the focus calcs

contents must also be explicitly marked as unfocusable during calc-only realize
operations in order to avoid triggering a full focus recalc which will error due
to missing focus adapter in the item block

Reviewed-by: Marcel Hollerbach <mail@marcel-hollerbach.de>
Differential Revision: https://phab.enlightenment.org/D10543