SVGHMI: fading page switch : defer update of classList to next animate() call when finishing transition.

classList update was happening in the same call as switch_page(), but this call is not meant to do any change in the DOM. This was triggering unwanted style and layout recomputation.

<?xml version='1.0' encoding='utf-8'?>
<BaseParams xmlns:xsd="http://www.w3.org/2001/XMLSchema" IEC_Channel="2" Name="py_ext_0"/>