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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{

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    "ID": "wamptest", 

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    "active": true, 

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    "protocolOptions": {

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        "autoPingInterval": 60, 

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        "autoPingTimeout": 20

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    }, 

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    "realm": "Automation", 

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    "url": "ws://127.0.0.1:8888"

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}