SL-20432 Filtering My Outfits with big number of items freezes UI

indra/llui/llaccordionctrltab.h

indra/llui/llflatlistview.cpp

indra/llui/llflatlistview.h

indra/newview/llinventoryitemslist.cpp

indra/newview/llinventoryitemslist.h

indra/newview/lloutfitgallery.cpp

indra/newview/lloutfitgallery.h

indra/newview/lloutfitslist.cpp

indra/newview/lloutfitslist.h

indra/newview/llpanelappearancetab.cpp

indra/newview/llpanelappearancetab.h

indra/newview/llpaneloutfitedit.cpp

indra/newview/llpaneloutfitsinventory.cpp

indra/newview/llpaneloutfitsinventory.h

indra/newview/llpanelwearing.cpp

indra/newview/llpanelwearing.h