kio_desktop: restore lost "slaveHandlesNotify" entry
42efaf44 accidentally dropped this entry as protocoltojson did not know about this key.
From what I understood, this is mainly needed for the use of the desktop protocol with the folder containment. There renaming a randomly placed file (icon) should not lose the placement information of the file (icon) due to the default KIO change signals.
While currently renaming a file does not change its position, that info seems lost though after a restart of the plasma shell. With this patch applied, the information is restored though.
First affected Plasma version that lost this bit would be 5.23. Someone with insight please backport as needed.
See also https://bugs.kde.org/show_bug.cgi?id=359596 for more background info on this flag.
Update: created respective bug at https://bugs.kde.org/show_bug.cgi?id=461793