[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.html

@@ -32,7 +32,7 @@
     <tr *ngFor="let row of data.getRows(); let idx = index" tabindex="0"
         class="alfresco-datatable__row"
         [class.alfresco-datatable__row--selected]="selectedRow === row"
-        [adf-upload]="allowDropFiles" [adf-upload-data]="row">
+        [adf-upload]="allowDropFiles && rowAllowsDrop(row)" [adf-upload-data]="row">
 
         <!-- Actions (left) -->
         <td *ngIf="actions && actionsPosition === 'left'" class="alfresco-datatable__actions-cell">