Form's handleMultiPart() method needs to be overridable to allow custom processing of multi-part streams

Currently handleMultiPart() method is declared "private final" in wicket.markup.html.form.Form class, and implementation of the uploaded parts of the multi-part stream on the server side.is tied to the File-type in the code, especially the code related to the FileUploadField class. Making handleMultiPart() "protected" instead of "private final" will allow custom "per form" multi-part HTTP POST stream processing. In particular, the processing will become possible, which is not tied to a particular implementation of the uploaded parts of the multi-part stream on the server side. It shall be noted in the Javadoc that overriding handleMultiPart() method will likely break FileUploadField logic. Also, it may be worth extracting the non-File-specific portion of the FileUploadField class logic into the base class (e.g., StreamUploadField), and advise using that base class instead of FileUploadField for non-File-specific uploads.