Avoid duplication of FrameworkListFileClass information in sfxprojs

[ViktorHofer]

I noticed in #4459 that the FrameworkListFileClass information is now duplicated in both:

• https://github.com/dotnet/windowsdesktop/blob/main/src/windowsdesktop/src/sfx/Microsoft.WindowsDesktop.App.Ref.sfxproj#L18 and

• windowsdesktop/src/windowsdesktop/src/sfx/Microsoft.WindowsDesktop.App.Runtime.sfxproj

  Line 17 in 2eaeedd

  <ItemGroup>

The items that are shared between both should be moved into the Directory.Build.props file in the same folder.

cc @LakshanF