Feature/#698 csharp80 upgrade

[Barsonax]

• Enabled C# 8.0 on all projects
• Changed appveyor image to use VS2019

Note that this does not yet enable nullable references. For that the following line needs to be added to the csproj's:

<NullableContextOptions>enable</NullableContextOptions>

The above could be done in a separate PR though since its also a different issue.

EDIT: Its now changed to:

<Nullable>enable</Nullable>

[Barsonax]

Solves #698

[Barsonax]

Seems we are getting a error in the appveyor build:

C:\projects\duality\Source\Core\Duality\Duality.csproj(479,11): error MSB4226: The imported project "C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Microsoft\Portable\v4.5\Microsoft.Portable.CSharp.targets" was not found

I believe this is because the VS2019 does not have the portable targeting pack installed. It should work if we install that. Locally it works atleast.

[Barsonax]

Jup portable is not even included anymore by default: https://stackoverflow.com/questions/54861759/build-of-pcl-lib-fails-with-new-vs2019-agent

So we have 3 options:

• Modify the visual studio installation with powershell
• Don't use C# 8.0 but use C# 7.3 (no nullable references...)
• Switch to .net standard

[ilexp]

I think we should take this opportunity to attempt option three and switch to .NET Standard first. If that turns out to be too massive, we can explore the other options. Let's keep this PR around for later, but continue discussion on #573.

[ilexp]

Needs conflict resolve after recent master upgrade to .NET 4.7.2 / C# 7.3 via #698

[ilexp]

Found out that we can't use C# 8.0 until we've also upgrade to .NET Core 3.0. Moved all related issues to a separate milestone so we can focus on .NET Standard 2.0 and C# 7.3 right now. Closing this.