It makes sense for explicit calls to Layout() to use the same logic as
is implicitly used when a TLW is resized, so override it to use
TLW-specific logic instead of using a separate DoLayout() for this.
Note that DoLayout() still has to be preserved because Debian code
search finds at least a couple of examples of its use outside the
library, meaning that there are probably quite a few more of them in the
wild.
Also, wxTopLevelWindow still needs its own wxEVT_SIZE handler because
the base class only calls Layout() if GetAutoLayout() is true, while we
want it to be always called. This might be worked around by just calling
SetAutoLayout(true) in wxTopLevelWindow ctor, but it seems safer, from
compatibility point of view, to keep wxTopLevelWindow::OnSize() instead.
See #18472.
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