Add Set rgb backlight command to 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 0-30, 0-30, 0-30 triplet.

Uses 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. Also includes a demo of this function OpenLCD_Qwiic_RGB_Backlight in Examples/Advanced directory.
Closes #9

[nseidle]

This looks fantastic! Thank you!