screen: disable locked settings

The settings for lock-delay and idle-delay are editable even if the corresponding dconf setting is locked.

Adding writeable bindings fixes this. I converted the screen-lock row into an expander row with the lock-delay row nested into it. This was done to avoid conflicting bindings to the "sensitive" property of the lock-delay row.

In the following recordings, both lock-delay and idle-delay are locked.

old_lock_behaviour new_lock_behaviour

Fixes #117