[kcms/autostart] Support enable and disable system autostarts.
Changes to the KCM:
- NoDisplay=True is always not shown in the KCM.
- Hidden field means it's disabled, will shown a checkbox to enable/disable it in the xdg autostart entry.
- Remove a desktop file that has a system version available, will revive the system version of desktop file.
Edited by Xuetian Weng