Implicit validation of the filemanager and filepicker form elements

In MDL-59443 a new feature was introduced allowing form elements to perform their own (so called implicit) validation without the need to add explicit rules into the form.

While working on making use of the new filetypes form element, it turned out that filemanager and filepicker elements could now make use of this new feature - without the need to put required validation to all the places.