[ADF-992] improve datatable styles according to UX guidelines (#2088)

* improve datatable style

* datatable and document list enhancements

- support to toggle header in DataTable
- Document List automatically hides header for empty content

ng2-components/ng2-alfresco-datatable/src/components/datatable/datatable.component.ts

@@ -71,6 +71,9 @@ export class DataTableComponent implements AfterContentInit, OnChanges, DoCheck
     @Input()
     rowStyleClass: string;
 
+    @Input()
+    showHeader: boolean = true;
+
     @Output()
     rowClick: EventEmitter<DataRowEvent> = new EventEmitter<DataRowEvent>();