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

docs/changes.txt

@@ -149,6 +149,11 @@ Changes in behaviour not resulting in compilation errors, please read this!
   new constructors taking wxSize for the gap argument is preferred. The same
   applies to wxFlexGridSizer as well.
 
+- wxBoxSizer::AddSpacer(x) now adds space in the direction of the sizer only
+  and not in both this and the orthogonal directions. This behaviour is what
+  most of the people expect but if you really relied on this overload adding
+  space in both directions you should change your code to use AddSpacer(x, x).
+
 
 Changes in behaviour which may result in compilation errors
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