Add playlist database based on the micropython 'btree' module.
Supported features are:
* Create playlist
* Load playlist
* Store position in playlist

Different playlist modes will be added in a followup for #24.

Implements #23.

The playlist data is stored in the btree database in a hierarchical
schema. The hierarchy levels are separated by the '/' character.
Currently, the schema is as follows: The top level for a playlist is the
'tag' id encoded as a hexadecimal string. Beneath this, the 'playlist'
key contains the elements in the playlist. The exact keys used for the
playlist entries are not specified, they are enumerated in native sort
order to build the playlist. When writing a playlist using the
playlistdb module, the keys are 000, 001, etc. by default.  The
'playlistpos' key is also located under the 'tag' key and stores the key
of the current playlist entry.

For example, a playlist with two entries 'a.mp3' and 'b.mp3' for a tag
with the id '00aa11bb22' would be stored in the following key/value
pairs in the btree db:
- 00aa11bb22/playlist/000: a.mp3
- 00aa11bb22/playlist/001: b.mp3
- 00aa11bb22/playlistpos: 000

Signed-off-by: Matthias Blankertz <matthias@blankertz.org>

@@ -80,1 +70,3 @@
-        self._play(self.playlist[self.playlist_pos])
+    def _set_playlist(self, tag: bytes):
+        self.playlist = self.playlist_db.getPlaylistForTag(tag)
+        self._play(self.playlist.getCurrentPath())