Added accent color support for brightness slider

[ZimM-LostPolygon]

Only for the main brightness slider for now, since it's the most used one:

[emoacht]

Thanks for your work.
But the color for brightness is intentionally fixed to differentiate the colors for range and contrast. So I will not accept this change as it is.
In addition, in the existing code base, the UI settings are handled in https://github.com/emoacht/Monitorian/blob/master/Source/Monitorian.Supplement/UIInformation.cs
and your code does not fit.

[ZimM-LostPolygon]

Right, I haven't noticed that existing code. I can refactor my PR.

However, I'm pretty sure that 95% of people do not even have the contrast slider enabled, and even if they do - it would be indistinguishable only if the chosen accent color is similar to the contrast slider color. And even then, it's not that hard to remember that contrast slider is always on the right. So to me personally it feels like a pretty good tradeoff.

[emoacht]

The accent color is fun but I doubt that the majority of Windows users are aware of and care about customization of accent color.

That said, if this feature can be incorporated, it will be an option to mitigate the confusion.

To capture accent color change, I added WindowPainter.ColorChanged event to
https://github.com/emoacht/Monitorian/blob/feature/Source/ScreenFrame/Painter/WindowPainter.cs
in feature branch. It can be used as controller.WindowPainter.ColorChanged in MainWindow.xaml.cs.
Then, the accent colors will need to be reflected when MainWindow initialized and when they are changed.

[emoacht]

Having seen no response, I have completed my own implementation for this purpose.
So this PR is closed.