No real changes, just add wxSizerItem::AddBorderToSize() helper.

Factor out this function from GetMinSizeWithBorder() as it will be used for
max size too in a next commit.

See #11497.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72344 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/sizer.h

@@ -436,6 +436,10 @@ protected:
     void DoSetSizer(wxSizer *sizer);
     void DoSetSpacer(const wxSize& size);
 
+    // Add the border specified for this item to the given size
+    // if it's != wxDefaultSize, just return wxDefaultSize otherwise.
+    wxSize AddBorderToSize(const wxSize& size) const;
+
     // discriminated union: depending on m_kind one of the fields is valid
     enum
     {