Choose a unique marker for mirrored shapes in export files

[svengoldberg]

Fix #1289

Task	Finished	Reviewer Approved
At least one test for the new functionality was added.	yes does not apply	OK
New classes have been added to the Python interface.	yes does not apply	OK
The code is properly documented with doxygen docstrings	yes does not apply	OK
Changes are documented at the top of ChangeLog.md	yes does not apply	OK

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.46%. Comparing base (bed1220) to head (b24f286).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1290   +/-   ##
=======================================
  Coverage   72.46%   72.46%           
=======================================
  Files         313      313           
  Lines       27395    27395           
=======================================
  Hits        19851    19851           
  Misses       7544     7544           

Flag	Coverage Δ
unittests	72.46% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/geometry/CTiglAbstractGeometricComponent.cpp	80.95% <100.00%> (ø)

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