Resolve "Make PersonStorage::Direction usage consistent"

Replaces usages of a raw int for the direction of deletion of a trajectory with the (already existing) enum PersonStorage::Direction. This enum was already used for some, but not all methods. With this MR, it is used in all methods of PersonStorage to be consistent.

Closes #465 (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()