Make Kickoff read-only when SystemImmutable

When widgets are SystemImmutable, as opposed to UserImmutable, this is because
the system was consciously locked-down by a system administrator, in this case
Kickoff should become essentially read-only, ie.:

• Favorites can no longer be added, removed, or re-arranged
• Applications can no longer be added as launchers to Task Manager
• Applications can no longer be edited or hidden or uninstalled
• The KMenuEdit shortcut is removed from the right-click menu

You can, however, still:

• Drag applications elsewhere to make eg. a desktop shortcut
• Access recent documents in the application context menus as well as jump list actions

CHANGELOG: Applications and favorites in Kickoff are now locked when widgets are locked by KIOSK restrictions

Differential Revision: https://phabricator.kde.org/D1609