Let `gL` deal with unvisited tabs instead of unread ones

Let `gL` deal with unvisited tabs instead of unread ones

Firefox considers a tab to be "unread" if it finishes loading in the background.
This means that if the current tab is loading, but very slowly, and you
therefore switch to another tab while waiting for it to finish, you can't use
`gl` to go back to the slowly-loading tab when it's done, because then it would
be marked as "unread", forcing you to use `gL` instead. This often tripped me
up.

This commit uses the notion of "unvisited" instead of "unread". This works much
better.