Reworked lock spell effect to use isLocked()
Fix bug #1348 (Evidence chest unlocks whenever stolen items are taken)

It's currently hardcoded to 50 to avoid a relocking exploit.
If I just used lock(), then the player could cast a lock spell at an unlocked evidence chest and permanently change its lock level.
Fix "Unlocked" tooltip displaying on doors/containers that were never locked.

Consider just not displaying this in the future.
Change one last check to isLocked()

At this point, setLockLevel(...) is never called directly, and getLockLevel() is only called in places where it returns 0 or a positive value.
Moved never lock check from miscextensions.cpp into the lock(...) function.

Thanks to Digmaster for this one.