Ability to go to landmark element in DOM inspector/DevTools

[matatk]

Ideally there would be an option to enable "developer mode" and add buttons to the pop-up that take the user to the corresponding landmark in the HTML tree view for the page. However this is not possible.

Another idea would be to include a Landmarks DevTools panel, though there are limitations in Firefox's support that may make this impractical at the moment.

[matatk]

If a DevTools page is created, it is possible to inspect certain elements from panels created in the DevTools window. This could mean another 'fork' of the pop-up code, designed to run in the DevTools, would be able to support this functionality by way of helper functions such as inspect()—though there are multiple security warnings there; need to check this will be accepted on AMO (haven't seen anything to suggest it won't be, but it may well be subject to more scrutiny).

This could go hand-in-hand with making a DevTools panel to implement #172, and maybe even some of #182.