Add background DSC classes for personalization

[Gijsreyn]

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• This pull request is related to an issue.

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This PR addresses issue #97. It contains the following new DSC classes:

• BackgroundPicture
• BackgroundSolidColor
• BackgroundSlideshow
• BackgroundWindowsSpotlight

Microsoft Reviewers: Open in CodeFlow

[Gijsreyn]

@denelon and @ryfu-msft, I thought of giving this one a go, but I ended up stumbling on an endless tour of scavenging through C# code. The BackgroundPicture, BackgroundSolidColor and BackgroundWindowsSpotlight should work on all OSes. However, when looking at the BackgroundSlideShow, it doesn't. I already pre-populated some functions to determine the OS it supports.

The most challenging part, which I couldn't solve, is this:

I searched the internet, and I saw a ton of workarounds to get the slideshow to work, all with their pros and cons, which are not very applicable for a DSC resource. I couldn't encrypt and decrypt this value correctly. If you have any suggestions, please let me know.

P.S. I'll update the issue with the remaining work.

[denelon]

@Gijsreyn, I'm playing catchup from being OOF last week. Also, ryfu is no longer at MSFT 😢. We'll have some other engineers joining in the fray here 😊.

[Gijsreyn]

Cool, that's a pity to hear 😢, but at the same time, looking forward to the others.