QSingleShotTimer: de-inline [4/4]: clean up includes

Include what you need, don't rely on transitive
includes. Forward-declare what you need in-name-only.

Also port the one unchanged include in the header
(qabstracteventdispatcher.h) to idiomatic form. It's an unrelated
change, but it looked so lost among the QtCore/ headers, I felt pity.

Amends e0573e7364.

Change-Id: I4f25df7465f08378bc4e003bde12c28f4d231f14
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>
(cherry picked from commit 49d5b0a5a10634841122bbbc5e986ce307e7017a)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/kernel/qsingleshottimer.cpp

@@ -4,13 +4,10 @@
 
 #include <QtCore/private/qsingleshottimer_p.h>
 
-#include "qabstracteventdispatcher.h"
 #include "qcoreapplication.h"
 #include "qmetaobject_p.h"
 #include "private/qnumeric_p.h"
 
-#include <chrono>
-
 QT_BEGIN_NAMESPACE
 
 QSingleShotTimer::QSingleShotTimer(Duration interval, Qt::TimerType timerType,

src/corelib/kernel/qsingleshottimer_p.h

@@ -16,12 +16,16 @@
 // We mean it.
 //
 
-#include "qabstracteventdispatcher.h"
-#include "qcoreapplication.h"
-#include "qmetaobject_p.h"
+#include <QtCore/qobject.h>
+#include <QtCore/qabstracteventdispatcher.h>
+#include <QtCore/qnamespace.h>
 
 QT_BEGIN_NAMESPACE
 
+namespace QtPrivate {
+class QSlotObjectBase;
+}
+
 class QSingleShotTimer : public QObject
 {
     Q_OBJECT