Add event onElementHealthChange

[znvjder]

This PR adds the serverside event onElementHealthChange

I think this is an insignificant change for many people, but it will certainly make life easier for ours scripters.

Refers to #3136

[Pirulax]

Hi, thanks for the PR!

SA changes entity health directly as well, so this implementation doesn't cover all cases.
The only way you could make this event work reliably is to dispatch it from the net code [since it tracks entity health for sync].

[znvjder]

Almost after a month's break, lol. I decided to use a server-side event only. It is the most accurate and insta calls when the health value changes. Works for player, ped, vehicle and object.

[Lpsd]

As @Pirulax mentioned, I think it would be nice for onElementHealthChange to trigger whenever element health changes, so not just via setElementHealth, but also when changed in-game (from fall damage, explosion, etc). There should be a parameter to the event for whether it was changed via script.

[znvjder]

Currently, regardless of the circumstances, this event will always be triggered. The parameter idea seems interesting.