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.
The PRs end up having merge conflicts anyway.  I suggest we do this, pay the cost now, and hopefully simplify life in the future.