reset loaded messages counts on new PLC program trasnfer

This fixes the problem that first message from newer PLC program maybe
lost, if it's issued before first PLC log fetching.

Closes #23

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