Fix healthbar HUD

[appgurueu]

Closes #979, closes #986

[LoneWolfHT]

Shall I close in favor of an engine fix as discussed in Discord?

[appgurueu]

Shall I close in favor of an engine fix as discussed in Discord?

If the engine gets a feature PR that allows implementing this slightly neater and the PR is merged, I will redo this.

[appgurueu]

This is how it looks now:

[appgurueu]

Last commit closes #986 by right-aligning the hearts.

[appgurueu]

Note that this legitimizes duplicating the HUD bar def, as it requires repositioning the health bar.

[appgurueu]

@savilli Bump. I see no reason not to merge this. This fixes two bugs. Implementing a builtin feature would be significantly more work (especially if inclusion in the engine is desired). This is now very clean and duplicates very little code (negligible). Pretty much everything that overrides the builtin defs has to duplicate just as much (small parts of the HUD element def). The only minor dirty thing I see is the use of the undocumented register_on_playerevent thing, which is properly annotated as a "hack".

[savilli]

LGTM

[appgurueu]

@LoneWolfHT please review