Enemy health bar disappears before damaging effect ends

When casting a long-lasting damaging spell the enemy's health bar should remain visible for the entire duration of the spell. Currently WindowManager::setEnemy is called once from spellcasting.cpp when the effect is first applied to the target. This call should either be moved to or duplicated in spelleffects.cpp when damage is dealt. Though it might be better to call setEnemy only once per frame somehow instead of once per effect per actor per frame.

Should probably also apply to healing effects.