Modernize DesignTools projects

[Nirmal4G]

Currently the DesignTools projects we have from the PR #3318 is based on the previous Design projects which were legacy project format. There's nothing wrong with it except that it is verbose. As many of the toolkit projects moved to Sdk-style. It's about time that the design projects be converted to a more modern project format that supports far more capabilities when used in VS IDE.

Previously, it was difficult to convert as there was an issue with .NET SDK v3 targets and VS IDE. Now that has been fixed, we can now convert them to Sdk-style. The design projects use WPF to show custom properties provider. Thus, we'll be using Microsoft.NET.Sdk.WindowsDesktop instead. Although, with .NET 5 we don't need to specify the desktop Sdk explicitly, it should also be usable from older v3 Sdk targets.

We want to force the WindowsDesktop SDK to always use WinFX targets from the SDK instead of using the .NET Framework's inbox version as the SDK version contains fixes around enabling PackageReference. In .NET SDK v5, we added an overridable option to always opt-into this behaviour. See dotnet/wpf#2976 and dotnet/wpf#4630

[ghost]

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