Should throw error on reading ill-formed .pet-file (e.g. wrong decimal separator)

We didn't check if the values from the pet file are parsed successfully, which lead to errors. This MR introduces helper methods that check that. I tried my best to divide my changes with commits so one can look at one commit at once when reviewing.

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()