Design discussion: dotnet/wpf#2869

The following changes need to be in order. In order to checkin without order (both side are compatible), I need to detect if props are imported twice. But that would double the code change. And there is no use once the flow is finished.

• SDK side change: Import Windows desktop target when property is set #12106
• Windows SDK side change: Muti target 5.0 second part wpf#3155
• SDK side tests (need both change in and update stage 0): Add test for Import windows props and targets #12107
• ensure this works with net5.0-windows when the change for getting -windows is in