UI enhancement to navigate to each XSS value in the response.

This is the 2nd half of the issue first raised in #2584. The referenced issue asks the XSS rules to indicate the number of times that a reflected XSS payload is found unmodified in the response.

This issue is to add a UI feature, once #2584 has been implemented, that supports the ability to easily navigate to each instance in the response. By default, when you select an XSS issue, it shows the response and scrolls to the location where the 1st instance exists (and highlights it), and this is great. If there are multiple instances, could some kind of Show Next/Show Previous buttons be added to the Alert tab so you could easily traverse through each additional place where its echoed in the response?

Start by reviewing issue #2584 and the existing Alert tab behavior when an XSS issue is selected. Trace how the response is displayed, how the first reflected instance is located and highlighted, and where the alert UI can expose navigation controls. Done means multiple echoed instances can be visited with Show Next and Show Previous while the selected instance remains highlighted.