[#5192, #5489] Allow configuration of initiative grouping

[Aioros]

This change adds two settings to the Initiative section of the combat configuration:

1. Roll Once per Creature: actors of the same type (roll formula - disposition - base actor) share a single initiative roll;
2. Group Combatants: actors of the same type with the same rolled initiative are grouped in the tracker (the current default functionality, but it can now be disabled).

Closes #5192
Closes #5489