Support for CPS-style (SDK-style) projects

Notes:

• With MSBuild 2019, the tools version has been changed from a version number (e.g. "15.0") to "current". It can therefor no longer be used to determine the tools set. However, it may be possible that the SDK-attribute can be used instead. We will need to determine if this can freeze the CSC.exe used. See also https://github.com/microsoft/msbuild/issues/5376 for my question on this subject

• Custom item types: https://github.com/dotnet/project-system/issues/113