File / Image field type

[gartner]

Build a new HTML_QuickForm-element for file / image uploads.

Use https://github.com/Widen/fine-uploader.

Should take the current image as an option, so it can be displayed as a preview in the form.
Will require an Ajax-endpoint to store temporary images. When an image is uploaded, return a url to the temporary image, for display in the element preview.
Store this url in the element, so that when submitted, the form logic can copy the selected image into the correct place.

Also needs a delete-button, which removes the preview.

All functions should only add info to the form-field about what was added to the form:
If deleted, flag that somehow, if new file added, give the temporary name. And then let the form logic handle adding the image permanently or deleting original image.