Enhanced media player implementation

[AlexViseven]

This merge request adds a persistent "queue" / playlist feature to the Nextcloud Media Player and implements a set of frequently requested playback behaviors and UI improvements:

• Shuffle mode (randomize playback order)
• Repeat modes (repeat one / repeat all / off)
• Automatically play the next file when the current media ends (so folders with mixed content will behave as playlists containing only media files)
• Automatic filtering to exclude non-media files when generating a playlist from a folder
• Improved media player notification controls while the app is minimized (skip to next / previous, play/pause)
  The proposed implementation has been used in the HiDrive application for a long time.

[AndyScherzinger]

pinged Tobias and Alper for a review - thanks for the contribution, looks great and interesting :)

@AlexViseven @dbotvin for the SPDX license identifier you should be using

AGPL-3.0-or-later OR GPL-2.0-only instead of GPL-2.0, the later is not a valid on per SPDX list and also wouldn't match the apps used license. You might want to add github.com/nextcloud/.github/blob/master/workflow-templates/reuse.yml to your fork or rebase to latest master branch of the nextcloud Android repo. Since the PR has conflicts you might want to rebase anyways.