waylandclipboard: roundtrip to get accurate focus state

The current approach is too complicated and not correct anyway.
To summarize QGuiApplication::focusWindow is delayed so the focus
watcher was introduced, however when the window is hidden and
setMimeData called before Wayland events have been processed,
then focusWatcher is naturally out of sync, so the check
was complicated to work around this state. However the same
issue arises when the window is deleted before calling
setMimeData.
Instead of introducing more complicated checks and workaround
roundtrip so the focusWatcher state is up to date before setting
the clipboard.
BUG:463199
FIXED-IN: 5.103


(cherry picked from commit dcb254dc)