Manual.rst: usage of drag and drop.

DragAndDrop.php, QuickFormQuery.php: bug fixes
FillStoreForm.php: no FE on a Form now ok - necessary for dnd sort forms.

DRAGANDDROP.md, PROTOCOL.md: Doc for "drag'n' drop" implementation.
dragAndDrop.php: API endpoint
DragAndDrop.php: Class for implementing drag'n' drop functionality.
Link.php: implement new renderMode=8 - returning only the sip.
QuickFormQuery.php: New entry point for processing "drag'n' drop".

delete.php: it seems that assign an array element to a string variable does not convert the string to an array. CR thought this was the case in the past (maybe a change in PHP7.2)

Html2Pdf.php: remove check if wkhtmltopdf is executable - without the check, it's possible to specify variables, like LD_LIBRARY_PATH, in front of the executable. The latest development version of wkhtmltopdf needs specific libssl and libcrypt libs.
Manual.rst: add a note the the latest wkhtmltopdf can be requested from the QFQ team.