Fix file encoding, add Open Source Code of Conduct, add back DesktopToasts

* Convert files from UTF-16 to UTF-8 or ASCII as appropriate,
  because core.autocrlf doesn't support UTF-16. Files that must
  remain UTF-16 are marked as binary to prevent core.autocrlf
  from trying to interpret them as UTF-8.
* Add Microsoft Open Source Code of Conduct statement.
* Add DesktopToasts sample back to this repo.