[#3029] Add 'concentration' object in actor system data

[krbz999]

This introduces a RollConfig field which shares some common data that all similar fields can have (eg initiative, skills, abilities).

The concentration property is for creatures only.

(Also fixes the documentation for the init property.)

  /*
   * @property {object} concentration
   * @property {string} concentration.ability   The ability used for concentration saving throws.
   * @property {string} concentration.bonus     The bonus provided to concentration saving throws.
   * @property {number} concentration.limit     The amount of items this actor can concentrate on.
   * @property {number} concentration.mode      The default advantage mode for this actor's concentration saving throws.
   * @property {object} concentration.roll
   * @property {number} concentration.roll.min  The minimum the d20 can roll.
   * @property {number} concentration.roll.max  The maximum the d20 can roll.
   */

[Fyorl]

Looks pretty good to me, just one comment

[Fyorl]

All good, thanks very much Zhell