improved uploading of files (#1730)

* improved uploading of files

- new core/UploadDirective to allow dropping files to any html element
- enhanced file dropping for DataTable rows (disabled by default)
- enhanced file dropping for DocumentList rows (disabled by default)
- upload drop area now handles file uploads for child elements (i.e.
rows in the document list)

* fix unit tests

* unit tests and code cleanup

* #1732, fix upload of folders

demo-shell-ng2/app/components/files/files.component.html

@@ -18,6 +18,7 @@
                 [currentFolderId]="currentFolderId"
                 [contextMenuActions]="true"
                 [contentActions]="true"
+                [allowDropFiles]="true"
                 (error)="onNavigationError($event)"
                 (success)="resetError()"
                 (preview)="showFile($event)">