Resolve "Refactor Petrack::UpdateImage"

Refactor Petrack::update image in a way, that it calls different methods for doing stuff. It is still a bit messy, because this barely changes the code itself.

This MR also adds more documentation and testing for a helper method.

closes #476 (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()