Add a way to listen for navigator events

[leonard84]

We currently use Groovy MOP interceptor magic to intercept calls to click, sendKeys, at and value, in order to record a screenshot before and after a click, as well as which element was acted upon.

While it certainly works, it would be better to have an officially supported way of hooking into these events.

[erdi]

Hey @leonard84, I'm currently working on this feature request. Would you please clarify why you are intercepting calls to at? Is it to act on page changes (as in "Page change listening") or do you actually need a callback before and after at checking for a page occurs? If it's the latter we could introduce a onAtCheckFailure which would allow users to implement whatever additional reporting they'd like to have on at check failures rendering #584 redundant.

[erdi]

@leonard84, FYI, I've extracted the bit about at into #590 as it doesn't have much to do with Navigator, really.

[aptester]

@erdi I would like to use the hooking mechanism requested by @leonard84 as well. Unlike @leonard84, I haven't figured out how to get Interceptor magic working to intercept those methods but as an intermediate step have defined a custom navigator which overrides those methods in the Navigator class. But with my approach, TemplateDerivedPageContent events can't be intercepted.