Remove caching of whether any of the children accept focus in
wxControlContainer as it can change at any moment under Mac, due to full
keyboard access being turned on and off (which can be done using
Ctrl-F7, i.e. easily, and so this might be something users actually do
and not just a theoretical edge case). This also incidentally fixes
caching of the wrong focusability state during window initialization,
when its children are not yet shown, as [NSView canBecomeKeyView:] used
for AcceptsFocusFromKeyboard() implementation under Mac, apparently
always returns false for hidden windows.
The behaviour under the other platforms should remain the same, but
AcceptsFocus() is slower now as it always has to query children instead
of being able to avoid doing it when we know that none of them accepts
focus anyhow. OTOH this only happens to the windows that don't accept
focus themselves and, at least at some moment, don't have any children
accepting focus neither, which should be quite rare and optimizing this
case doesn't seem to be worth the extra code complexity due to extra
preprocessor platform checks.
Closes#18089.
198a4e97a0