add script to check license header

Add a script for checking if every file starts with a license header.
This python script compares the first x characters of every file matching the filters to a copied license.
I copied the license from a source file.

Parameters yet to choose: Directories and file endings to check (currently include, src, tests and h, cpp, py).

The output of the script first prints the amount of files not matching the license followed by a detailed comparison.
If errors exist, the script exits with code 1.

Open Questions:

• Does exit code 1 make the pipeline fail?
• Currently, about 20 source-files are not matching the license string. I noticed that it may be because, the encoding of some files is not UTF-8 but UTF-8 with BOM which adds some leading characters and thus break this check. What should we do about this?

ToDo after merge:

• add to pipeline

closes #227 (closed)

Reviewer Checklist

Formatting

• the pre-build checks succeed

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()