Player ghost cosmetics fixes, refactoring

[tehkab]

Some (WIP) improvements to player ghost cosmetics (crown, amulet, more to come?) and other player ghost tracking

• Moving player ghost related code from utils.lua to a separate utils/player_ghosts.lua. This gets included by utils.lua, so shouldn't change much in itself.
• Caching player's player-ghost entity id in the player's entry in the PlayerList table. This allows quick access to the ghost entity without having to iterate over all (up to 90!) ghost entities every time we do something
• Storing the player-ghost cosmetic flags in the same PlayerList table entry. This allows player-ghosts to get their crowns and amulets back when you hide and re-show ghosts. Fixes Investigate hiding player ghosts and showing them again may lose crown/amulet states #82
• Wands no longer disappear when hiding and re-showing ghosts either

Need to periodically (and/or on change/request) re-send cosmetic information; ghost cosmetics and held wand also disappear when mod restarting (and we lose track of them in the PlayerList) Let's leave resyncing PlayerList state after a mod restart to another PR for now?