Jump in video feature

Keyboard shortcut to jump in video. I had a bug which stopped the video image from updating again, where f.ex. the video slider suggests that the video is playing, but no new frame is shown. It should hopefully be fixed now

Reviewer Checklist

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
• documentation updated (if applicable)

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()

Closes #505