Add support for loading/saving m3u playlists

[teamcons]

Fixes #515
And a 11 years old bug: https://bugs.launchpad.net/elementaryos/+bug/1308876

Ctrl+S allows saving the playlist, but i am not sure how to expose it in the UI. I included it so half the support isnt in Needs Design Hell.

the implementation skip extended M3U flags. Meaning Extm3u playlists will still work.
URL will be skipped, so the coverage is alas not complete

when looping through the files and upon finding a m3u file in the list, Application.loop_through_files just call the support method that knows how to read m3u, and replace it with an array of all the files in it
it is then up to playback to check what in there actually exists

Saving M3U pretty much just loops through the queue to put it all in a file.

[zeebok]

In my testing, I saved a playlist from Music but it failed to open. I am going to look into it more because I am not sure if the issue is how it is saving or how it is loading.

[teamcons]

In my testing, I saved a playlist from Music but it failed to open. I am going to look into it more because I am not sure if the issue is how it is saving or how it is loading.

Hmmm.... It worked in my testing. I will test more cases/see if i can get issues

[teamcons]

My access to a PC will be very limited in the coming weeks, i will not be able to take care of this for a couple weeks

[teamcons]

Vague thought but once this is merged, technically we could bin the whole "save/restore queue in dconf" and simply save/load from a m3u file in the data folder

That would avoid two separate code blocks for the same thing (less moving parts), and skip dumping shit of dubious length in dconf

maybe also not relying on dconf but straight up loading a file could also speed up recovery.

Saving the playlist only on app closing (or DE session disconnecting if the apps are allowed grace time) would also reduce disk writes and cpu usage