[ADF-1595] Typeahead - Fetch the value from the process variables (#2388)

* Get the typeahead value from the process variables. We should not use the processDefinition to get again all the values

* Improve the dropdown widget

ng2-components/ng2-activiti-form/src/components/widgets/dropdown/dropdown.widget.ts

@@ -105,4 +105,8 @@ export class DropdownWidgetComponent extends WidgetComponent implements OnInit {
         this.logService.error(error);
     }
 
+    isReadOnlyType(): boolean {
+        return this.field.type === 'readonly' ? true : false;
+    }
+
 }