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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BEREMIZ_OBJS = $(BEREMIZSRC:.c=.o)

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all: warn some_binary

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	@echo "*** all done ***"

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warn:

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	@echo "*** Sample Makefile, does nothing ***"

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some_binary: $(BEREMIZ_OBJS)

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	@echo "* Would link $^ -> $@"

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%.o: %.c

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	@echo "* Would compile $< -> $@"