Windows: Return false from event processing of unhandled multimedia keys.

Otherwise, potentially active players no longer receive the keys when
a Qt application is running.

Task-number: QTBUG-43343
Change-Id: Iefa511a101734690305e3244fafec4a460a9212d
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>

src/plugins/platforms/windows/qwindowskeymapper.cpp

@@ -833,7 +833,10 @@ bool QWindowsKeyMapper::translateMultimediaKeyEventInternal(QWindow *window, con
 
     const int qtKey = CmdTbl[cmd];
     sendExtendedPressRelease(receiver, qtKey, Qt::KeyboardModifier(state), 0, 0, 0);
-    return true;
+    // QTBUG-43343: Make sure to return false if Qt does not handle the key, otherwise,
+    // the keys are not passed to the active media player.
+    const QKeySequence sequence(Qt::Modifier(state) + qtKey);
+    return QGuiApplicationPrivate::instance()->shortcutMap.hasShortcutForKeySequence(sequence);
 #else
     Q_UNREACHABLE();
     return false;