QWindowPrivate: de-inline dtor and ctor

It's private API, but exported, so de-inline the dtor to pin the
vtable in QtGui instead of potentially duplicating it in every library
that uses the class.

Ditto ctor, but that's just code hygiene: we don't want the code to be
duplicated across all users.

Pick-to: 6.3
Task-number: QTBUG-45582
Change-Id: I91ea38be20fc67795466a68ca5721837255b33a0
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/gui/kernel/qwindow.cpp

@@ -236,6 +236,12 @@ QWindow::~QWindow()
     d->isWindow = false;
 }
 
+QWindowPrivate::QWindowPrivate()
+    = default;
+
+QWindowPrivate::~QWindowPrivate()
+    = default;
+
 void QWindowPrivate::init(QScreen *targetScreen)
 {
     Q_Q(QWindow);

src/gui/kernel/qwindow_p.h

@@ -73,6 +73,9 @@ public:
         WindowFrameExclusive
     };
 
+    QWindowPrivate();
+    ~QWindowPrivate() override;
+
     void init(QScreen *targetScreen = nullptr);
 
 #ifndef QT_NO_CURSOR