Turn off backlight when sleeping, not just display

[tomaae]

Is your feature request related to a problem? Please describe.
Option "Turn Screen Off While Sleeping" turns off display, but not backlight.

Describe the solution you'd like
Since there are many types of displays which are handled differently, I propose following solution:
Add 2 more options to config:

1. Execute command in cli when screen is turned off
2. Execute command in cli when screen is turned on

For example in my case it would be:

1. gpio -g pwm 19 0
2. gpio -g pwm 19 40

[UnchartedBull]

I added this to #410 as all the screensaver improvements will be tracked there.

[whoim2]

I solved this by connecting the backlight to the power-on pin (plug-in for octoprint) in parallel with the power supply relay, to an unused pair of contacts

[tomaae]

Thats not an option with my display unfortunately. I use full header display and messing with that would void warranty. I rather avoid that since its not cheap display.