SVGHMI: better handling of page switch fade-out.

- In cases or relative jump to current page, page was keeping black after switch.
- Now keep track of undergoing page switch without comparing subscribed page to visible page.

<?xml version='1.0' encoding='utf-8'?>
<BeremizRoot URI_location="LOCAL://">
  <TargetType/>
  <Libraries Enable_SVGHMI_Library="true"/>
</BeremizRoot>