Add property to allow multiple gesture recognizers

To enable other gesture recognisers that might be affected by the
UIPanGestureRecognizer instances used for opening and closing a drawer,
like the swipe to delete UIPanGestureRecognizer on UITableViewCell
instances that have UITableViewCellEditingStyle ==
UITableViewCellEditingStyleDelete, implement the
UIGestureRecognizerDelegate
shouldRecognizeSimultaneouslyWithGestureRecognizer method.