[FEATURE] Support recursive directory structure in playlist view

[randoragon]

I have a lot of playlists. For ease of use I would like to categorize them into subdirectories. However, ncmpcpp ignores directories in the playlist view, which means I'm forced to keep all the playlists together.

I would like for ncmpcpp to display directories in the playlist view and allow for stepping in and out of them recursively. The same functionality that's already present in the file browser view.