Unload loaded video when loading pet with empty SRC

• When loading empty src
  • Reset Animation
  • Update Sequence
  • Reset mImg, mImage and mFilteredImage (i.e. all images saved in Petrack)
  • Fade In LogoItem
• Change LogoItem to use QPropertyAnimation for fading
• Change updateSceneRect to properly shrink sceneRect when loading empty sequence

Feedback needed: Fading animation speed, currently 2 seconds.

Closes #375 (closed)

Related !260 (merged)

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()