Move reusable code from mobile to common

This will be useful for a future desktop application.

common/lib/src/playback.dart

@@ -0,0 +1,96 @@
+import 'package:meta/meta.dart';
+import 'package:rxdart/rxdart.dart';
+
+import 'library.dart';
+
+/// Base class for Musicus playback.
+abstract class MusicusPlayback {
+  /// Whether the player is active.
+  ///
+  /// This means, that there is at least one item in the queue and the playback
+  /// service is ready to play.
+  final active = BehaviorSubject.seeded(false);
+
+  /// The current playlist.
+  ///
+  /// If the player is not active, this will be an empty list.
+  final playlist = BehaviorSubject.seeded(<InternalTrack>[]);
+
+  /// Index of the currently played (or paused) track within the playlist.
+  ///
+  /// This will be zero, if the player is not active!
+  final currentIndex = BehaviorSubject.seeded(0);
+
+  /// The currently played track.
+  ///
+  /// This will be null, if there is no  current track.
+  final currentTrack = BehaviorSubject<InternalTrack>.seeded(null);
+
+  /// Whether we are currently playing or not.
+  ///
+  /// This will be false, if the player is not active.
+  final playing = BehaviorSubject.seeded(false);
+
+  /// Current playback position.
+  ///
+  /// If the player is not active, this will default to zero.
+  final position = BehaviorSubject.seeded(const Duration());
+
+  /// Duration of the current track.
+  ///
+  /// If the player is not active, the duration will default to 1 s.
+  final duration = BehaviorSubject.seeded(const Duration(seconds: 1));
+
+  /// Playback position normalized to the range from zero to one.
+  final normalizedPosition = BehaviorSubject.seeded(0.0);
+
+  /// Initialize the player.
+  /// 
+  /// This will be called after the database was initialized.
+  Future<void> setup();
+
+  /// Add a list of tracks to the players playlist.
+  Future<void> addTracks(List<InternalTrack> tracks);
+
+  /// Remove the track at [index] from the playlist.
+  Future<void> removeTrack(int index);
+
+  /// Toggle whether the player is playing or paused.
+  Future<void> playPause();
+
+  /// Seek to [pos], which is a value between (and including) zero and one.
+  Future<void> seekTo(double pos);
+
+  /// Skip to the previous track in the playlist.
+  Future<void> skipToPrevious();
+
+  /// Play the next track in the playlist.
+  Future<void> skipToNext();
+
+  /// Switch to the track with the index [index] in the playlist.
+  Future<void> skipTo(int index);
+
+  /// Set all values to their default.
+  void reset() {
+    active.add(false);
+    playlist.add([]);
+    currentTrack.add(null);
+    playing.add(false);
+    position.add(const Duration());
+    duration.add(const Duration(seconds: 1));
+    normalizedPosition.add(0.0);
+  }
+
+  /// Tidy up.
+  @mustCallSuper
+  void dispose() {
+    active.close();
+    playlist.close();
+    currentIndex.close();
+    currentTrack.close();
+    playing.close();
+    position.close();
+    duration.close();
+    normalizedPosition.close();
+  }
+}