Utils: More forward declarations / drop unused headers [P-R]

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

Change-Id: I01fb80b93164a7fe4c0bd2cd8f9e4f27523efefa
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/libs/utils/persistentsettings.cpp

@@ -25,6 +25,9 @@
 
 #include "persistentsettings.h"
 
+#include "fileutils.h"
+
+#include <QCoreApplication>
 #include <QDateTime>
 #include <QDebug>
 #include <QDir>