Ensure that message boxes with only "OK" can be closed with Escape in wxMSW.

Ensure that message boxes with only "OK" can be closed with Escape in wxMSW.

The native task dialog doesn't allow using Escape (nor Alt-F4 but this is less
annoying) to close it unless it has a Cancel button, so by default the dialogs
with only "OK" couldn't be closed with Escape.

Work around this by creating a Cancel button with "OK" label instead. This is
not ideal but there doesn't seem to be any other way to make this work.

See #12501.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67620 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775