Split up `addToGalleryACleanUp` and move it into PreparationBloc

**Is your feature request related to a problem? Please describe.**
If users create an image/audio/video file but decide not to submit it after the encoding process, it will be in their gallery and may irritate them, I don't think it will happen often, but nobody knows what users do ; )

**Describe the solution you'd like**
Split the function up into cleanup and adding to the gallery, and run it in the SubmissionBloc.
(Maybe the splitting is not needed, wiping the directory could also be fine for nonaudio files, in case the user wants to stop the operation)

**Describe alternatives you've considered**
Just let it be

**Additional context**
If for some reason anyone else except me takes this on please reach out so we can discuss what my half-baked thought is.