Refactor grid item

This MR does some slight refactoring of the GridItem. It is a bit of preparation for the MR getting the alignment grid box out of Control.

Strong recommondation to look at each commit on its own and to disable the diff for whitespace changes.

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()