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/advanceddockingsystem/dockmanager.cpp

@@ -49,6 +49,7 @@
 #include "workspacedialog.h"
 
 #include <utils/algorithm.h>
+#include <utils/fileutils.h>
 #include <utils/qtcassert.h>
 
 #include <algorithm>