implement `LocalMusicLibrary`

[Mouayad Alhamwi]

What needs to be done:

• LocalMusicLibraryFacade: contains only a public interface which can be used by other components. Has a LocalMusicRepository

• AudioLibraryScanner (with unit tests).

• Implement the following functions in LocalMusicLibraryFacade:
  - addMusicDirectory
  - removeMusicDirectory

[Mouayad Alhamwi]

• Turns out a LocalMusicLibraryRepository was not required and it only added an extra layer of complexity,
• Now LocalMusicLibraryFacade manages the ops of:
  • loading previously stored music library.
  • add a list of tracks to the music library.

[Mouayad Alhamwi]

• Finished unit-testing the following functionalities of LocalMusicLibraryFacade:
  • getLibrary
  • getAlbums
  • getArtists
  • getTracks
  • addTracksToLibrary

[Mouayad Alhamwi]

Implementing AudioLibraryScanner to search for audio files in a given directory(folder):

• Option 1: read all files in a directory using dart:io package => then use metadata_god to read each file metadate => use Isolate.run to save the track cover to the app's direcotry => return the list of tracks when finished.

• Option 2: add the previously described process to a custom package built upon metadata_god . This option may give us a performance advantage. especially with saving files in the backgroud.
  Note: check if we can save a group of files once instead of one each time.

[Mouayad Alhamwi]

Updates on

Implementing AudioLibraryScanner:

• handle the search for music files in a given directory via AudioFilesScanner; a mixin to give us only audio files in a
  directory and its sub-directories.
• extract a [BaseTrack] from a file via BaseTrackFromFileExtractor.
• in an Isolate, saves the extracted image into the application directory.
• add unit tests for AudioFileScanner.
• add unit tests for AudioLibraryScanner's image saving feature.

[Mouayad Alhamwi]

• in an Isolate, saves the extracted image into the application directory.

For now, we won't be using an Isolate for saving the image since it appears to take more time.
TODO: test this again in an integration test