Tab list hiding

Leaves a player out of chosen viewers' tab list. Only the tab-list row goes. The player stays visible in the world and can be messaged and targeted by commands.

Start it with the hidden_from_tablist power, the hide_from_tablist action, or /opp tablist hide. A player always sees themselves.

• It only changes those viewers' screens. The server's player list stays the same.
• Viewers are checked every half second.
• As with name obfuscation, the Social Interactions screen and name tab-completion still show the player.