Add drop-in Playlist support

[CrashCringle12]

This PR is split from and dependent upon the implementation of the Sort Menu discussed here: #585.

...this also adds support for drop-in playlists in the Playlists folder.
Playlists are just txt files (exact format as favorites) placed in the Playlists folder. All available playlists will be shown under the Playlist category and will load the respective listing as preferred sorting. This can aid users who wish to distribute their favorites or recommendations, hold small tournaments, or just want separate listings for groups of songs.

Attaching the same video since it shows Playlists in use at the end.

SubSort2.mp4

[CrashCringle12]

(Still in "Other" directory currently, but can/will move after discussing where it should go and other implementation stuffs)

[teejusb]

(Still in "Other" directory currently, but can/will move after discussing where it should go and other implementation stuffs)

Yeah! So I think we have two options.

1. Other directory implies that the only those with machine access can really load the playlists, since otherwise they wouldn't have the perms to add files there. Maybe that is fine and we can limit the feature to that.

2. If we don't do Other then the next place to consider would be in the local profiles. This is public friendly as one can modify their profiles via usb.

I think in both cases there should be some logic to determine what we do if all the songs aren't available on the machine. I would say we should either not display the playlist or display an empty one with a caution sign saying not all songs are found on the playlist. Another option would be display the songs that are found. I'm not sure out of these we should go with but open to hearing thoughts.

[stclairdaniel]

(Still in "Other" directory currently, but can/will move after discussing where it should go and other implementation
I think in both cases there should be some logic to determine what we do if all the songs aren't available on the machine. I would say we should either not display the playlist or display an empty one with a caution sign saying not all songs are found on the playlist. Another option would be display the songs that are found. I'm not sure out of these we should go with but open to hearing thoughts.

I think most preferable is a message saying not all songs are found and listing the ones which aren't found.