Set RGB Backlight in one command without flicker

[fourstix]

Add function to update RGB backlight as a single settings command followed by 3 bytes for the red, green and blue values. This will eliminate the delays and flickering of using 3 commands to set the colors, one by one.

This command has a few advantages over the original.

1. It changes the backlight color at once, with no flickering through intermediate colors.
2. It's quicker and a bit more intuitive to use a single command.
3. It allows for all rgb value combinations, not just those that map to a 0-30, 0-30, 0-30 triplet. (Probably not a big deal, as maybe only an artist who has mastered like Monet, might be able to notice the difference.)

[fourstix]

@nseidle I used the available command byte 43 '+' and added a new mode to consume the next 3 bytes for the red, green and blue values and set the backlight without any delay or confirmation message. I also included a demo of this function OpenLCD_Qwiic_RGB_Backlight in Examples/Advanced directory. I will submit a pull request with these changes. If you have any suggestions or would like to see anything done differently, please just let me know.