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

include/wx/sizer.h

@@ -510,7 +510,7 @@ public:
     wxSizerItem* Add( int width, int height, const wxSizerFlags& flags);
     wxSizerItem* Add( wxSizerItem *item);
 
-    wxSizerItem* AddSpacer(int size);
+    virtual wxSizerItem *AddSpacer(int size);
     wxSizerItem* AddStretchSpacer(int prop = 1);
 
     wxSizerItem* Insert(size_t index,
@@ -913,6 +913,8 @@ public:
                       wxT("invalid value for wxBoxSizer orientation") );
     }
 
+    virtual wxSizerItem *AddSpacer(int size);
+
     int GetOrientation() const { return m_orient; }
 
     bool IsVertical() const { return m_orient == wxVERTICAL; }