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.

image: skvorl/beremiz-requirements

pipelines:
  default:
    - parallel:
        - step:
            name: Codestyle checks
            script:
              - ln -s /CanFestival-3 $BITBUCKET_CLONE_DIR/../CanFestival-3
              - /usr/bin/python --version
              - ./tests/tools/check_source.sh
        - step:
            name: Application tests
            max-time: 10
            script:
              - ln -s /CanFestival-3 $BITBUCKET_CLONE_DIR/../CanFestival-3
              - ./tests/tools/run_python_tests.sh