Update project files

[Jonarw]

Proposed changes:

• Update csproj files to VS2019 format
• Change target of OxyPlot.SharpDX to netstandard1.1 (minimum required to use SharpDX)
• Change OxyPlot.SharpDX.Wpf to target both net452 and netcoreapp3.1
• Change SimpleDemo to target netcoreapp3.1
• Make CacherRenderContext public, remove InternalsVisibleTo.cs. The additional encapsulation provided by this does not seem worth the effort for me, IDE support by VS2019 also seems wonky
• Update OxyPlot.Core reference to 2.0.0 stable

Probably this will make some changes to the CI setup necessary. Unfortunately I have next to no experience with this kind of stuff. @objorke Do you agree with my proposed changes? If I implemented this, could you help with the CI setup?

[objorke]

This sounds very good. I can try to help with the CI part - preferably with Azure Pipelines.