Add kcron to systemsettings

1) kcm_cron.desktop has X-KDE-System-Settings-Parent-Category=system-administration,
but there is no category system-administration in systemsettings5
changed this to systemsettings5 category session as already patched by debian
patch by Jeremy Lainé

2) kcron is a kcm, therefore the docbook should be installed in subdir kcontrol like all other kcm docbooks
BUG:347028
REVIEW:128033