Adds docstrings to backend and frontend modules. In addition this PR also
Adds a hash size parameter to the get_file_hash function
Updates some error messages thrown by functions from the backend. Now these error messages are more informative and appropriate
refactors/simplifies the filter_public_models_table function
Makes typing of percentages parameters be tuples and removes corresponding length checks
Adds an extra check to the _yt_download helper function. This check raises an error when a youtube link is valid but does not lead to any content
Fixes a bug where choosing "aac" as output format leads to "m4a" output format
Reparametrizes the helper function _mix_audio so that it takes one parameter for each input audio file instead of a list containing all input audio files
Reparametrizes _mak_song_dir so that its control flow is a bit more logical
Refactors (and renames) get_song_cover_name_harness so that parameter update_key becomes update_placeholder which is a boolean indicating whether only the given textbox placeholder should be updated
renames several other functions (and some variables) in both frontend and backend modules so they are consistent with docstring descriptions
Adds docstrings to backend and frontend modules. In addition this PR also
get_file_hashfunctionfilter_public_models_tablefunctionpercentagesparameters be tuples and removes corresponding length checks_yt_downloadhelper function. This check raises an error when a youtube link is valid but does not lead to any content_mix_audioso that it takes one parameter for each input audio file instead of a list containing all input audio files_mak_song_dirso that its control flow is a bit more logicalget_song_cover_name_harnessso that parameterupdate_keybecomesupdate_placeholderwhich is a boolean indicating whether only the given textbox placeholder should be updated