[xjtznglu] Screensaver locks input

Create a screensaver which merely locks the keyboard and mouse. The display is unaffected, so can continue displaying (and updating) whatever it was displaying. This is for when one walks away from one's computer, but there's nothing secret on the screen for a passerby to read, but we do not wish a passerby to interact with the computer.

This is a modern screensaver.