[qtwayland] Signal clipboard change when window unfocus clears the co…

…ntent. Fixes JB#46506

QWaylandWindow::unfocus() clears the offer and refocus later will get
a new one, but only if there's something in the clipboard.
Clipboard cleared between those two failed to signal any change to
application side.

src/client/qwaylanddatadevice.cpp

@@ -80,6 +80,7 @@ QWaylandDataOffer *QWaylandDataDevice::selectionOffer() const
 void QWaylandDataDevice::invalidateSelectionOffer()
 {
     m_selectionOffer.reset();
+    QGuiApplicationPrivate::platformIntegration()->clipboard()->emitChanged(QClipboard::Clipboard);
 }
 
 QWaylandDataSource *QWaylandDataDevice::selectionSource() const