add functions for locking/unlocking the corona

Summary:
add global immutability/setImmutability in scripting
there was a locked property in containment, but
didn't make much sense as the lock/unlock state is
always global. a global function would be better.

containment.locked may also be changed to lock the corona instead
(doesn't seem reliable now) but should probably be deprecated.

Test Plan:
correctly locked nd unlocked widgets from the console.
i'm not sure wether allowing systemimmutable as is dangerous and not
easily reversible, but does have an use for first
time setup of kiosk environments

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5009