Utils: Continue Environment/EnvironmentChange consolidation

Make Environment a stack of changes that gets "expanded" to
a full environment before things are actively accessed.

Later this expansion should be done lazily if possible.

Task-number: QTCREATORBUG-28357
Change-Id: If1c7bfdb9f58b81e71c51ed87ee75d6964a47019
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/libs/utils/filepath.h

@@ -32,7 +32,6 @@ namespace Utils {
 
 class DeviceFileAccess;
 class Environment;
-class EnvironmentChange;
 enum class FileStreamHandle;
 
 template <class ...Args> using Continuation = std::function<void(Args...)>;