Write intermediate AssemblyInfo

[dittodhole]

Currently, the GenerateAssemblyInfo property is used (with Version, VersionPrefix, VersionSuffix, PackageVersion, InformationalVersion, AssemblyVersion, and FileVersion property) to emit the version.
This should be reworked into a GenerateAssemblyInfo-independant approach (ie generating a intermediate AssemblyInfo.g.cs file), so to be compatible to legacy csproj and consistent emitting of the version, regardless of the GenerateAssemblyInfo-value.