Add active state to subnav menu item when triggered with mouse click.

[StevenDufresne]

Originally reported in https://meta.trac.wordpress.org/ticket/7491, by pitamdey.

Add an indicator for the anchor tag when a dropdown menu is active and triggered by a mouse click.

Screenshot

[ryelle]

Just noting, that screenshot is the focus state, I'm guessing it's meant as a suggestion. When you tab away, you still loose that "selected" state.

I don't think this is an accessibility requirement, so it could probably be fairly minor. We definitely don't want anything that a user could confuse with the focus state.