In ckeditor's image picker, selected upload image was cleared when choosing a different target folder

Reproduction path:

1. In CMS, open a document containing rich text field with ckeditor.
2. Click Image icon on the toolbar to insert a image.
3. Select an image from the upload panel, the selected image file name is shown next to the "Choose file" button.
4. Select to different target folder for upload at the folder treeview in the left side.

Expectation: the selected image is retained after changing target folder (tested at https://cms.demo.onehippo.com)
Actual: selected image is cleared.

Cause: The new single-file-upload in CMS7-9197 is ajax-based, in which operations prior uploading (click upload button) are done at the client side. When changing the target folder (an ajax click), the server side component (wicket) refreshes the file-upload widget. It will clears all client-side states (e.g. selected upload image).