Introduce new method `isBeingShown` to query whether the tooltip is already being presented

Introduce new method `isBeingShown` to query whether the tooltip is
already being presented. For example, this helps in case you want to
present the tooltip by single tapping another UIView object. If you
simply call `presentPointingAtView`, it will rebuild the entire view
and re-animate its presentation.

Please, let me know if you have a better idea to accomplish the same :-)