Power Display: add option to control SDR content brightness when display is in HDR

### Description of the new feature / enhancement

This would let users control SDR brightness when HDR is enabled on display.
Basically the slider "SDR content brightness" in System > Display > HDR of windows setting.

### Scenario when this would be used?

When I enable HDR, the desktop and lots of other apps will still use SDR.
Therefore the current slider will not change the display brightness, even HDR content will not have its brightness changed at all (tested with youtube video). So basically the current slider is useless with HDR on.

### Supporting information

The app "Twinkle Tray" already has this feature and it works very well.

Start by locating Power Display's existing brightness-control entry point and researching the Windows HDR behavior described in the issue, including the SDR content brightness setting. Compare the expected behavior with the referenced Twinkle Tray feature; done means users can control SDR brightness while HDR is enabled without losing the existing display-brightness behavior.

Written by the indexing model from the issue text.