Add dotnet core 3.0 support

[punker76]

Describe the changes you have made to improve this project

Support DotNet Core 3.0

• Add target netcoreapp3.0 and use FrameworkReference Microsoft.DesktopUI for this
• Disable paket (doesn't support netcoreapp3.0) and use package references
• Change the SDK from MSBuild.Sdk.Extras to Microsoft.NET.Sdk.Wpf
• Remove the sdk from global.json (so it takes the latest)
• Add installing the SDK to appveyor.yml
• Run all other targets (net45, net46, net47)

[Lakritzator]

It's nice to see you already took the initiative, I unfortunately didn't have time yet.
Don't forget to replace build.ps1, your version is deeply integrated with paket.

If you need help, let me know!!!!!

P.S.
One thing I do need to mention, currently the dotnet core 3.0 SDK can some times be unstable!!
Using the latest might break your build...