Feature/gh 22/persistent bars

[tajobe]

Adds an always-on healthbar option for NAME (mob) and SCOREBOARD (player) healthbar configs via special-case duration config.

This has a couple implications requiring follow-up:

• We aren't processing all entity damage events, only damage by entities. This is fine normally because a healthbar only shows for a few seconds when damaged from an entity, but for permanent bars damage like falling or fire really should keep the bar up to date. One complication is it shouldn't create a bar, just update one if showing. See Healthbars not updated from non-entity damage #29.
• Given healthbars will let you see mobs through the world, I imagine one might want to limit which mobs it applies to. See Mob filtering for healthbars #30.

That said, they could be used as-is if desired.

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Completes #22