vNext (.NET Standard 2.x support)

[mauroservienti]

Related to https://github.com/RadicalFx/housekeeping/issues/72

.NET Core 3.0 will support WPF, we cannot expect all users to move to NET Core 3.0 straight away and at the same time we cannot reliably support multiple versions of Radical. The best move seems to be moving Radical to netstandard2 so that it can be consumed by NET Core and Full Framework libraries.

• [x] prep move https://github.com/RadicalFx/Radical/pull/257
  • move output path to common folder to not rely on post build scripts
  • drop GitVersion in favor of MinVer
• [x] drop all what is obsolete now https://github.com/RadicalFx/Radical/pull/259
• [x] eval if multi-targeting net452 and netstandard2 is an option
• [x] move to NET Standard 2 and drop all what NET Standard 2 doesn’t support https://github.com/RadicalFx/Radical/pull/260
• [x] evaluate which breaking changes to introduce
• [x] drop all #if directives not anymore needed
• Obsolite types that, despite being supported in NET Standard 2, won’t make it for v3.0
• [x] Add SourceLink support
• [x] Switch build script to SimpleExec and align with latest changes
• [x] remove ico and use the one from the main website
• [x] move to release flow https://github.com/RadicalFx/Radical/pull/263

[mauroservienti]

Removed the requirement to obsolete types, it'll be done in 2.1.0. As beta is already available on Nuget this can be considered as done.