I remember to have seen a similar question in the QlikCommunity a few days back, though I am unable to locate it now. The solution proposed in that post was something similar to the one in the attached file.
Not selecting any item will imply selecting all items. However if you have a list box with "Always select one value" checked and if you want to select more than one item, the workaround used in the attached solution will be handy.
