Tour step flow tracking

[IGreatlyDislikeJavascript]

Legacy issue from original Tour.

Currently, no proper tracking of tour steps is performed. This means use of Prev/Next is just a simple numerical jump backwards and forwards, instead of a true move to the previous (and next) tour steps.

This is a slight edge case. In most use cases this doesn't manifest as a problem, however if a tour uses non-trivial logic in callbacks (onNext, onShow etc) along with tour.goTo to maniplate the flow of a tour during "tour time", the Prev/Next buttons may not operate in a way the user - or the tour implementor - is expecting.

The solution is to properly implement tour step tracking by pushing the step index to a tracking array on each tour.showStep(), and popping on each tour.prev() call.

However further funcs need to be exposed to allow calling code to manipulate the tour step tracking array, so that step flow after use of tour.goTo() and similar can be managed.