Add icon for AudioStreamPlaylist

[Meorge]

This PR adds an icon for AudioStreamPlaylist, based off of the icons for the file-specific AudioStream types and AudioStreamPolyphonic:

I was a bit conflicted on whether to use the file-specific AudioStream icon (i.e., with the music note) or have the playlist "sub-icon" (three horizontal lines) take up the whole space of the icon. AudioStreamPolyphonic uses the music note background despite not being tied to a single file format, which was my main reason for choosing it. If there is further reasoning behind why the music note background would make sense for AudioStreamPlaylist, then I can certainly update the icon to not use it.