add override option for "-force-d3d11" (for unity 6000.x new projects)

unity defaults to dx12 now.. but editor flickers and is (more) sluggish.

could set this value using https://github.com/unitycoder/UnityInitializeProject
but then need to restart editor..

could have checkbox in settings,
OR  maybe better in New Project dialog,
"[x] use dx11" (and it remembers the setting, for 6000.x projects?)