General file info interface for datatypes

*Title: General file info interface for datatypes

Incentive:
Make it easier to find out file information for a given attribute/datatype.
Instead having to know the datatype that has file information we instead
want to query the datatype interface if it has file information and if so
return that information.

*Documentation:
Some new methods are introduced to the eZDatatype interface, one for checking
for file info support and one for the actual information

methods:
 - hasStoredFileInformation(): Returns true if the datatype supports this
 - storedFileInformation(): Returns the information as an array or false if no info was found
   The structure looks like:
   * filepath
   * filename (optional)
   * original_filename (optional)
   * filesize (optional)
   * mime_type (optional)
   All optional fields will be calculated if missing.

Supported datatypes:
 - ezimage: Returns original image
 - ezbinaryfile: Uses stored binaryfile
 - ezmedia: Uses stored mediafile

*Testing:

*SVN Revisions:
8734