Power display - groups

### Description of the new feature / enhancement

I'd like to be able to group the monitors into a single item so i can have one slider to change the brightness of all monitors.

### Scenario when this would be used?

Changing the brightness / turning off all of the displays at once. I have three displays and when i want to change the brightness on them i have to change every slider, if i could group all of them it would be easier to change the brightness.

### Supporting information

Something like what Twinke Tray has.

No files, tests, or entry points are named. Start by locating the PowerToys display-brightness controls and reviewing how individual monitor sliders and power actions are handled. Done means users can group monitors and change brightness or turn off every display through one control, with the behavior reflected in the UI.

Written by the indexing model from the issue text.