Multiple & Advanced Upload:

- fix problem when more than one file uploaded - there was only one variable {{_filename}}, which was not sufficient for more than one upload element per form. New: {{_filename_<FE_NAME>:FE:allbut}}.
- implemented {{_deleted_<FE_NAME>:F0}}: if exist and is '1', than this means that the previous uploaded file has been trashed through the user by clicking on the trash.

Index.rst: update doc. Some more notes are needed. See one of the upcomming commits.
Constants.php: Renamed CLIENT_UPLOAD_FILENAME and created CLIENT_FILE_DELETED.
Save.php: Set var CLIENT_FILE_DELETED and use renamed CLIENT_UPLOAD_FILENAME.