Make from_msecs a QTimer class-static again

It will need to be called from removed_api.cpp once we widen
QTimer::singleShot() from ms to ns.

Don't use the QAbstractEventDispatcher::Duration typedef in qtimer.h
to avoid including the resp. header. Keep the typedef as the return
value on the definition of from_msecs(), though, as a static assertion
that they're identical.

Task-number: QTBUG-128426
Change-Id: Ib59895f4441e92813fe39778bb3836b039fdfbf0
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 65a249715941bf6b6ecc980d3f865f367f66522d)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/kernel/qtimer.cpp

@@ -290,7 +290,8 @@ void QTimer::timerEvent(QTimerEvent *e)
     }
 }
 
-static QAbstractEventDispatcher::Duration from_msecs(std::chrono::milliseconds ms)
+QAbstractEventDispatcher::Duration // statically asserts that Duration is nanoseconds
+QTimer::from_msecs(std::chrono::milliseconds ms)
 {
     using Duration = QAbstractEventDispatcher::Duration;
 

src/corelib/kernel/qtimer.h

@@ -144,6 +144,8 @@ private:
     Q_DECLARE_PRIVATE(QTimer)
     friend class QChronoTimer;
 
+    static std::chrono::nanoseconds from_msecs(std::chrono::milliseconds);
+
     inline int startTimer(int){ return -1;}
     inline void killTimer(int){}