Resolve "Refactoring of findCodeMarker-method"

Closes #368 (closed)

Refactored findCodeMarker-method. Return detected points in a separate list instead of appending to the CrossList. Also resolve some clang remarks.

For now, the lines where this function is called just append the return list to the crossList afterwards. But this functionality can be further used when refactoring refineWithAruco because the detected codes get further processed there (this is currently in progress in #152 (closed) when also using detected candidates).

Reviewer Checklist

General code quality

• naming conventions are met (see .clang-tidy for detailed information)
• no static analyzer warnings in new code parts (e.g., use clang-tidy for checking)

General usability

• old versions of pet-files are still loadable

Only if changes in UI

• new elements are also saved and loaded from pet-file
• check if tab order is still correct
• all new SpinBoxes are promoted
• new keybindings added to Petrack::keyBindings()