waylandclipboard: Be a bit smarter about when to use the QGuiApplication::clipboard

The newly introduced keyboardFocusWatcher suffers from a similar problem on keyboard leave as the one it fixed on keyboard enter. When Qt hides a window, the focusWatcher and itself think that the app has focus because keyboard_leave did not arrive yet but setting the normal clipboard will still fail. This is an issue in klipper as it shows this pattern of window hide and afterwards setting the clipboard. To workaround this we can check if the focusWindow is visible even though we still think we should have focus. BUG:462979 FIXED-IN:5.102