Added support for Central Package Management

[Dryadxon]

• Added solution directory tree traverse to find the Directory.Packages.props file when AvaloniaVersion is null. This is needed if the version is defined only globally, and not overridden in the csproj.
• Added check for the presence of the PackageVersion tag. This is needed since the Directory.Packages.props could be the only place where the Version attribute is defined.
• Added check for the presence of the VersionOverride attribute. This is needed since the version used by a project could still be different from the global one, at least in theory.

These changes allow the application to support the Central Package Management feature introduced with NuGet 6.2 / .NET SDK 6.0.300. I didn't add checks for the validity of either the csproj or Directory.Packages.props, I assume they are both valid since, to allow this application to work correctly, the opened project / solution needs to be built successfully. Besides, I think it isn't this project responsibility to validate the format of these files, but the MSBuild one.

[kuiperzone]

Done. Thanks again.

[Dryadxon]

Thanks to you for this fantastic software, I am glad to be able to contribute, even if just a little 😅