QObject: Make it clear we don't install duplicated event filters

Pick-to: 6.6 6.5 6.2 5.15
Change-Id: I3048b50700880dd2445a5a65823fef02b196ce7d
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
(cherry picked from commit 43de84644292978f2b8bce209830cf14d5384904)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/kernel/qobject.cpp

@@ -2310,6 +2310,9 @@ void QObjectPrivate::setParent_helper(QObject *o)
     If multiple event filters are installed on a single object, the
     filter that was installed last is activated first.
 
+    If \a filterObj has already been installed for this object,
+    this function moves it so it acts as if it was installed last.
+
     Here's a \c KeyPressEater class that eats the key presses of its
     monitored objects: