Implements #12085. Add doc and custom error message. Finalize directDownload notation. Add unit test.

WIP: Implements #12085 Persistent download links. 1) Doku fehlt, 2) Nur eine SQL in der Config ist vermutlich viel zu wenig (es fehlt ein Konzept), 3) AS _link Class sollte diese Modus auch unterstuetzen, 4) symlink dl.php muss aktuell noch manuell angelegt werden.

Bug Fixes #11925. Form.parameter.downloadButton might contain now all parameter, incl. d, G, ... . KeyValueStringParser::explodeEscape() has been refactored. Function Link->paramPriority() has been renamed/rewritten to Link->paramPreparation(): double fire of explode() eliminated, code should be more efficient. Several new/enhanced unit tests.

Implements #6793. Source files for ZIP archives might now specified with a path/filename how they are called inside the ZIP.