Change wxBoxSizer::AddSpacer() to only add space in sizer direction.

Change wxBoxSizer::AddSpacer() to only add space in sizer direction.

It used to add a spacer with the given size in both directions but this was
counter-intuitive and wasn't expected even by the original author of this code
so change it to behave more reasonably.

Closes #11197.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61939 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775