prevent writing of mixed line endings in ProjectFileUpdater

The ProjectFileUpdater works internally with Unix line endings.
The file content is now converted to and from Unix line endings
according to the original content of the file.

This fixes TestApi::changeContent on Windows.

Change-Id: Id4be2829260d919e750d2642d8fbc3a6dc67c535
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>