ItemExists
bool ItemExists(int32 id, DeskbarShelf shelf);
bool ItemExists(const char* name, DeskbarShelf shelf);
ItemExists(message);
exists = fBarView->ItemExists(id, shelf);
exists = fBarView->ItemExists(name, shelf);
void ItemExists(BMessage* message);
int32 choice = fParent->ItemExists(*item, state.destination,
int32 ItemExists(PackageItem& item,