Dissolution of marriage — Settlement agreement — Alimony — Modification — Amount — Need — Imputed income — Trial court’s imputation of income to former wife when determining amount of modified alimony was in contravention of terms of parties’ marital settlement agreement in which former husband agreed to pay unemployed wife a specific amount of durational alimony for 8 years, modifiable if husband became involuntarily unemployed, and did not contemplate or require wife to seek employment after the dissolution of marriage