Provide options for the "system environment" used in Qt Creator

On macOS it is pretty hard to set the system environment for UI applications (like Qt Creator) when run through the Dock, Spotlight or Finder.

The same is true even in Ubuntu, where applications started through "Desktop" entries (their "Dock" or the search) are not run in the same environment as a shell in a terminal.

So provide an environment setting for Qt Creator "globally", e.g. in Preferences > Environment > System.