Centralize HUD updating in update_builtin_statbars.
Fixes race condition in 'on joinplayer' that causes stat bar bugs.
Correctly scales stat bars to custom max values set by mods.
* Respect object property hp_max field for players
This allows modders to configure the maximal HP per player
* Statbars: Downscale bar to full 20 HP when exceeding this value
Add default max HP for players and breath constants to builtin
Document the constants
* Rename PLAYER_MAX_HP -> PLAYER_MAX_HP_DEFAULT
This unifies the settings APIs.
This also unifies the sync and async registration APIs, since the async
registration API did not support adding non-functions to the API table.
