Move m_labelWin to wxStaticBoxBase itself

It will be reused by all platforms and is not specific to wxGTK.

This also means WXDestroyWithoutChildren() doesn't need to be virtual
any longer.

src/common/statboxcmn.cpp

@@ -31,6 +31,8 @@ extern WXDLLEXPORT_DATA(const char) wxStaticBoxNameStr[] = "groupBox";
 
 wxStaticBoxBase::wxStaticBoxBase()
 {
+    m_labelWin = NULL;
+
 #ifndef __WXGTK__
     m_container.DisableSelfFocus();
 #endif
@@ -46,7 +48,15 @@ void wxStaticBoxBase::WXDestroyWithoutChildren()
           i != children.end();
           ++i )
     {
-        (*i)->Reparent(parent);
+        // The label window doesn't count as our child, it's really a part of
+        // static box itself and it makes no sense to leave it alive when the
+        // box is destroyed, so do it even when it's supposed to be destroyed
+        // without destroying its children -- by not reparenting it, we ensure
+        // that it's destroyed when this object itself is below.
+        if ( *i != m_labelWin )
+        {
+            (*i)->Reparent(parent);
+        }
     }
 
     delete this;