Locator bar hiding

Leaves a player off chosen viewers' locator bar from Player Locator Plus (2.2.0 for 1.20). Their mark is gone from that bar. Nothing else changes.

Start it with the hidden_from_locator power, the hide_from_locator action, or /tma locator hide. A player always sees themselves.

• Needs Player Locator Plus on the server. Without it, these do nothing.
• It only changes those viewers' bars. Player Locator Plus's own hiding (sneaking, pumpkins, invisibility) still works on top of it.
• Viewers are checked every half second. A player who relogs is hidden from the moment they join.