BibDocFile: added subformat support

• subformat is a new way to add more semantic to the traditional concept of file extension. A file can exist in different formats, e.g. a picture can be saved as a .jpeg or a .gif. A subformat is a way to give more information to this, e.g. a picture can be saved as a .gif;icon or a document can be in the form of a .pdf;ocr. This means that the file becomes a particular instance of a more abstract document, and with the subformat it's possible to give a better idea of what is the intended use of the particular file instance (e.g. to be used as an icon, or to be considered as the OCRed derived format of an original scanned PDF).

• Here the subformat is introduced at any level of Invenio, in BibDocFile, WebSubmit functions, BibFormat elements, BibUpload FFT.

• The traditional special treatment given to icon has now been merged into the more generela framework of subformat. Still backward compatibile code has been kept, although for old records, a migration kit should be run.