Utils: More forward declarations / drop unused headers [D-G]

Round 1 - focus on headers.
For classes with initial in range [D-G].
Try to keep the same separators between different kind of headers.

Change-Id: Id42cd7b3743a816f75cecb6b576c0e37cb057473
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: hjk <hjk@qt.io>

src/libs/utils/fileutils.cpp

@@ -33,6 +33,8 @@
 #include <QDebug>
 #include <QOperatingSystemVersion>
 #include <QTextStream>
+#include <QXmlStreamWriter>
+
 #include <qplatformdefs.h>
 
 #ifdef QT_GUI_LIB