Re-enable T4 generation in build

[stevenbrix]

There's a bug in the VisualStudio T4 text engine that causes CI and local builds to fail. We will need to either wait for 16.1 Preview4, or add a reference to Newtonsoft.Json ourselves to unblock ourselves and then remove the following lines from PresentationCore.csproj, WindowsBase.csproj, and PresentationFramework.csproj:

<!-- <Import Project="$(WpfCodeGenDir)DesignTimeTextTemplating.targets" /> -->

Other option is to convert to using mono/t4