Resolve "Quit PeTrack"

If PeTrack is closed, while the video plays, the player just continues. Adding setAttribute(Qt::WA_DeleteOnClose); to the constructor of Petrack fixes this issue (by deleting Player upon closing).

Steps to reproduce error:

• Open Video
• ActivateLoop (View > Loop)
• Start Video
• Close PeTrack (without pausing video!)

Bug: PeTrack continues to play the video (visible e.g. in the TaskManager)

This MR should resolve this bug.

Closes #399 (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()