Fix an exception on wx-3.0-gtk3 in PLCOpenEditor when generating ST files.

On Python2.7 with WX3.0 and GTK3, an assertionError rises on generating a ST file if the name is already set. The first generation works without problems, if you generate the file a second one, PLCOpenEditor tries to open the file
save dialog with the pre entered name from last run. Then the following assertion pops up:

PyAssertionError: C++ assertion "volDummy.empty() && pathDummy.empty()" failed at ./src/common/filename.cpp(568) in Assign(): the file name shouldn't contain the path

This fix reduces the filepath of the ST file the to the filename. Now it works fine.

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<?xml version='1.0' encoding='utf-8'?>

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<PyFile xmlns:xhtml="http://www.w3.org/1999/xhtml" xmlns:xsd="http://www.w3.org/2001/XMLSchema">

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  <variables/>

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  <globals>

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    <xhtml:p><![CDATA[]]></xhtml:p>

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  </globals>

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  <init>

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    <xhtml:p><![CDATA[]]></xhtml:p>

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  </init>

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  <cleanup>

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    <xhtml:p><![CDATA[]]></xhtml:p>

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  </cleanup>

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  <start>

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    <xhtml:p><![CDATA[]]></xhtml:p>

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  </start>

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  <stop>

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    <xhtml:p><![CDATA[]]></xhtml:p>

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  </stop>

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</PyFile>