Fix: Method 'get_FetchMarkdownDescriptionAsync' does not have an implementation.

[loop8ack]

Next fix for a Visual Studio 17.7.0 Preview 1.0 update. The following changes have been made:

• The "Microsoft.VisualStudio.*.dll" files have been updated.
• The NuGet packages have been updated.
• The GalleryEntry class has been updated for the new version.

[madskristensen]

Thanks