Add basic track editor

This replaces the recording selector in the main menu.

lib/editors/tracks.dart

@@ -0,0 +1,106 @@
+import 'package:flutter/material.dart';
+
+import '../backend.dart';
+import '../database.dart';
+import '../selectors/files.dart';
+import '../selectors/recording.dart';
+import '../widgets/recording_tile.dart';
+
+class TrackModel {
+  String path;
+
+  TrackModel(this.path);
+}
+
+class TracksEditor extends StatefulWidget {
+  @override
+  _TracksEditorState createState() => _TracksEditorState();
+}
+
+class _TracksEditorState extends State<TracksEditor> {
+  int recordingId;
+  List<TrackModel> tracks = [];
+
+  @override
+  Widget build(BuildContext context) {
+    final backend = Backend.of(context);
+
+    return Scaffold(
+      appBar: AppBar(
+        title: Text('Tracks'),
+        actions: <Widget>[
+          FlatButton(
+            child: Text('DONE'),
+            onPressed: () async {
+              // TODO: Save tracks.
+            },
+          ),
+        ],
+      ),
+      body: ListView(
+        children: <Widget>[
+          recordingId != null
+              ? RecordingTile(
+                  recordingId: recordingId,
+                  onTap: selectRecording,
+                )
+              : ListTile(
+                  title: Text('Select recording'),
+                  onTap: selectRecording,
+                ),
+          ListTile(
+            title: Text('Files'),
+            trailing: IconButton(
+              icon: const Icon(Icons.add),
+              onPressed: () async {
+                final Set<String> paths = await Navigator.push(
+                  context,
+                  MaterialPageRoute(
+                    builder: (context) => FilesSelector(
+                      baseDirectory: backend.musicLibraryPath,
+                    ),
+                  ),
+                );
+
+                if (paths != null) {
+                  setState(() {
+                    for (final path in paths) {
+                      tracks.add(TrackModel(path));
+                    }
+                  });
+                }
+              },
+            ),
+          ),
+          for (final track in tracks)
+            ListTile(
+              title: Text(track.path),
+              trailing: IconButton(
+                icon: const Icon(Icons.delete),
+                onPressed: () {
+                  setState(() {
+                    tracks.remove(track);
+                  });
+                },
+              ),
+            ),
+        ],
+      ),
+    );
+  }
+
+  void selectRecording() async {
+    final Recording recording = await Navigator.push(
+      context,
+      MaterialPageRoute(
+        builder: (context) => RecordingsSelector(),
+      ),
+    );
+
+    if (recording != null) {
+      setState(() {
+        recordingId = recording.id;
+      });
+    }
+  }
+}