Enhancement: 2 frames for parents-christmas icon instead of 1

[DuckBergen]

What is your suggestion, and why should it be implemented?

for people who want to add a frame when parents-christmas's health gets low, it isnt even hard
from

animation.add('parents-christmas', [17], 0, false, isPlayer);

to

animation.add('parents-christmas', [17, 18], 0, false, isPlayer);

in HealthIcon.hx

[treacherousfiend]

Already fixed (but not yet merged) in #614