Add clicker that can open URLs in new tabs.

Unlike in Chrome, simulating Cmd+click mouse event won't open a link in
a new tab in Firefox. Therefore, some matching needs to be done when
attempting to simulate this feature. The down side is that now you can't
simulate an actual Cmd+click on any element with a href in them.
Add more matching to new tab links
Use safe window. Deactivate on exit.