M3u support suggestion

[ryonakano]

@teamcons I created a suggestion PR for elementary#813 instead of leaving detailed replies for each comment.

Changes Summary

• Separate frontend and backend (resolves Add support for loading/saving m3u playlists elementary/music#813 (comment))
• Fix indentation (resolves Add support for loading/saving m3u playlists elementary/music#813 (comment))
• Handle error when saving playlist (resolves Add support for loading/saving m3u playlists elementary/music#813 (comment))
• Use regex to check file suffix
• fix: Do not return incomplete File array if error
• Do not use print for debug/warning logs
• style: Remove unnecessary else and newlines
• Remove unnecessary cast

[teamcons]

Yes, it works
thank you! (i feel ashamed for the bad code)