[ADF-621] show drop effect on folders only (#1897)

* show drop effect on folders only

- fix `hasValue` api for data rows (avoid 'false' value to be evaluated as missing value)
- support for evaluating drop support for rows
- document list enables upload zones for folders only

* api improvements as per code review

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

@@ -264,4 +264,8 @@ export class DataTableComponent implements AfterContentInit, OnChanges {
             this.executeRowAction.emit(new DataRowActionEvent(row, action));
         }
     }
+
+    rowAllowsDrop(row: DataRow): boolean {
+        return row.isDropTarget === true;
+    }
 }