Port KConfig away from KStandardDirs:

* use QStandardPaths enum instead of resource name in the API, and
internally.

* Modify KAutostart to use the xdg config autostart subdir, rather than
the KStandardDirs-only "autostart" resource (which contained both KDE
and XDG dirs). This removes the KDE-specific autostart dir, which will
help simplifying the KCM.

* These API changes also propagate into KDesktopFile and KSharedConfig.