Separate blank and lock

[confluence]

I wanted to decouple blanking the screen from locking the screen, because I would like to blank to save power if I step away from the computer briefly, but I don't want to lock the screen automatically until much later.

This PR uses the previous "blank" preference for blanking, and adds a new separate preference for locking.

I also made it possible to lock first and blank later, in case someone has the opposite use case. (This requires noblank to be set with xset, because the default behaviour is to blank). Sometimes I see a crosshatch pattern before the xsecurelock saver kicks in, but this appears to be a random (?) glitch and not something that powerkit is responsible for. Most of the time the saver is displayed immediately, as expected.