create_shared_thumbnail_path

Creates the shared thumbnail path for the file that corresponds to the given uri and size.

No checks are made regarding the existence of the thumbnail.

It is the duty of the caller to free the returned string.

Parameters:

uri	the uri of the file whose shared thumbnail we want to find.
size	the thumbnail size (e.g. normal, large).

Returns:

a string with the thumbnail path or NULL if the uri could not be converted to a local filename.