Added "displaylock" and "displaywake" commands to lock / wake the screen

[ndbroadbent]

You can also send displaylock_sleep as the payload to also call commandDisplaySleep and turn off the display. (Otherwise it shows the password field for a little while. And a bit annoying if we need to send two commands via mqtt.)

EDIT: I also figured out how to wake up the screen - https://apple.stackexchange.com/a/237551/56850