Issue #881

Specify optional value for List(selection:).

This keeps selection on macOS, but not on iPad. On iPad each row in the List needs to be NavigationLink, no need for .tag. The selection is not updated, need to manually update with onTapGesture

struct Sidebaer: View {
    class ViewModel: ObservableObject {
        @Published var group: BookGroup?
    }

    @StateObject private var vm = ViewModel()

    var body: some View {
        List(selection: $vm.group) {
            allView
            foldersView
            tagsView
        }