[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/data/datatable-adapter.ts

@@ -32,6 +32,7 @@ export interface DataTableAdapter {
 
 export interface DataRow {
     isSelected: boolean;
+    isDropTarget?: boolean;
     hasValue(key: string): boolean;
     getValue(key: string): any;
 }