Resolve "Custom buttons in PMessageBox"

This MR adds the functionality to add custom buttons to PMessageBoxes

Returns the clicked button index (0 for the 1st one, 1 for the 2nd...)

Left an example usage in the code to make testing easier -> shown when opening an older pet file

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()