Add mouseover and mouseout commands

Added LinkHints.activateModeToHover and LinkHints.unhoverLast to
perform hovering. These are unbound by default.

Also call LinkHints.unhoverLast automatically before hovering/
clicking the next link, to avoid permanently showing hover effects
(eg. IMDb watchlist grid view mode).

Fixes #484, #1089.
I've been thinking about doing this for a while, but had been worrying that it would make other PRs have merge conflicts. Now that you've officially sanctioned it, I'll update my PRs :)