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.
We should consider reformatting these such that each is on its own line.

Currently, pretty much every PR that affects the commands has to be merged by hand.  This would be much easier if they were one per line.