added the option to select a folder on the editablefilefield

this fix adds the option to select a folder. I tried to do it with a TreeDropdownField, but that one didn't work. A treedropdownfield needs a form, because it tries to call the 'tree' action. I used a dropdownfield instead.

There is also an issue for this pull request:
https://github.com/silverstripe/silverstripe-userforms/issues/20