Decrease the size of the message box test dialog in the sample.

This dialog was getting too tall, especially in the rich message boxes case,
so reduce its vertical size.

Also centre it on screen to prevent it from being displayed partially off it
because contents was added to the dialog after creating it.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76255 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Show the result of closing rich message box in the sample.

Improve showing the button which closed the message box by doing it inside the
dialog used for testing it instead of popping up a separate message box just
for this.

And do the same thing for rich message boxes as for the normal ones.

See #16153.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76256 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Fix return value of wxRichMessageDialog with only an "OK" button.

Apply the same workaround as was applied in r67771 to wxMessageDialog to
wxRichMessageDialog and map IDCANCEL returned from it to IDOK if "Cancel" is
the only button in the dialog -- as in this case it's really just a renamed
(for convenience of allowing the user to use "Esc" to close it) "OK".

Closes #16153.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76257 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775