timestamp update in .pro.user file

For now the third line of .pro.user file is updated on every opening of a .pro project file even if there is no change of the project file itself.

So it would be better only update .pro.user file if really settings have been changed by user. So it is better for Concurrent Versions System like SVN.

Snippet from some .pro.user file. The Timestamp in the comment is updated on every startup of the project through Qt Creator.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE QtCreatorProject>
<!-- Written by Qt Creator 2.4.1, 2012-09-15T03:08:42. -->
<qtcreator>