Dissolution of marriage — Child custody — Modification — Final judgment of dissolution of marriage established parties’ rights with respect to shared parental responsibility and timesharing and could not thereafter be modified without findings that the modification was in the best interests of the child — Although former wife was not child’s biological mother, language of final judgment stating that the parties “shall have shared parental responsibility for the minor children” and that former wife had “majority timesharing with the children” established former wife as a parent for the purpose of the final judgment — Order granting former husband’s supplemental petition for modification disregarded res judicata effect of final judgment and modified parental responsibilities and parenting plan/timesharing established in the final judgment without findings required by statute