some haptic feedback on entering multi-select mode

deltachat-ios/View/ContactCell.swift

@@ -219,6 +219,8 @@ class ContactCell: UITableViewCell {
         if sender.state == UIGestureRecognizer.State.began,
            let tableView = self.superview as? UITableView,
            let indexPath = tableView.indexPath(for: self) {
+            let generator = UIImpactFeedbackGenerator(style: .medium)
+            generator.impactOccurred()
             delegate?.onLongTap(at: indexPath)
         }
     }