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).
Move a private variable inside a do function closure
Merge pull request #1032 from mrmr1993/mouseover

Add mouseover and mouseout commands