WIP .NET core migration
Convert PxCoco and Prexonite to .NET Core 2.2 projects.
Deleted source files that were not part of the build.
Remove version numbers from AssemblyInfo (managed by dotnet CLI)
Re-add license.txt for PxCoco (GPL with generated code exemption)
Set PxCoco up as a NuGet package.
Add CI and release jobs for PxCoco.