Fix filtered Add All when there are already some items selected.
Fix filtered Add All when there are already some items selected.

HomesteadCabinet/multiselect,michael/multiselect,kidaa/multiselect,HomesteadCabinet/multiselect,loki315zx/multiselect,loki315zx/multiselect,kidaa/multiselect,Sylvain/groupedmultiselect,toxalot/multiselect,michael/multiselect