macOS: Resend event if the first input method event changes focus

QAbstractItemView opens the editor and changes focus in response to the
first QEvent::InputMethod event. Changing focus however cancels the
current composition, so the key press is lost and needs to be repeated by
the user. What QAbstractItemView does is probably not that unique, so
fixing this there - if possible at all - is not a general solution.

In the platform specific input method handling code, we can generally
discover that the focus object changed in response to the InputMethod
event that gets sent to the focus object. On macOS, we can then pass the
key event to the input method once more, which works fine.

On Windows and X11, the situation is more complicated. That the focus
changed is easily discovered, but re-sending the key press in such a way
that the input method responds doesn't seem possible. On Windows, using
SendInput simply delivers the exact key to the focus object, without
starting the expected composition.

On X11 with IBus, it should be possible to call ProcessKeyEvent again on
the IBus input context when we discover in filterEventFinished that the
focus object changed while delivering the event. However, any attempt to
do so has no effect.

So, for now only fixing this for macOS.

Task-number: QTBUG-54848
Change-Id: I0f7b786cfa57430e6a1e7d605415265cf53eb675
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

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