Add wxSizerFlags::DisableConsistencyChecks()

This allows to (hopefully temporarily) disable size flag check asserts.

docs/changes.txt

@@ -25,7 +25,9 @@ Changes in behaviour not resulting in compilation errors
   (it used to succeed in wxMSW).
 
 - Using invalid flags with wxBoxSizer or wxGridSizer items now triggers asserts
-  when done from the code or error messages when done in XRC.
+  when done from the code or error messages when done in XRC. These asserts are
+  best avoided by fixing the flags, but wxSizerFlags::DisableConsistencyChecks()
+  can be used to globally suppress them until this can be done.
 
 - wxWS_EX_VALIDATE_RECURSIVELY is now the default behaviour, i.e. calling
   Validate() or TransferData{From,To}Window() will now also call the same