Add playback position slider to program screen

ProgramScreen became a StatefulWidget. It listens to playback position
updates from the backend and updates the slider unless the user is
currently interacting with it.

lib/screens/program.dart

@@ -1,6 +1,39 @@
+import 'dart:async';
+
 import 'package:flutter/material.dart';
 
-class ProgramScreen extends StatelessWidget {
+import '../backend.dart';
+
+class ProgramScreen extends StatefulWidget {
+  @override
+  _ProgramScreenState createState() => _ProgramScreenState();
+}
+
+class _ProgramScreenState extends State<ProgramScreen> {
+  Backend backend;
+  StreamSubscription<double> positionSubscription;
+  double position = 0.0;
+  bool seeking = false;
+
+  @override
+  void didChangeDependencies() {
+    super.didChangeDependencies();
+
+    backend = Backend.of(context);
+
+    if (positionSubscription != null) {
+      positionSubscription.cancel();
+    }
+
+    positionSubscription = backend.position.listen((pos) {
+      if (!seeking) {
+        setState(() {
+          position = pos;
+        });
+      }
+    });
+  }
+
   @override
   Widget build(BuildContext context) {
     return Scaffold(
@@ -11,6 +44,34 @@ class ProgramScreen extends StatelessWidget {
         ),
         title: Text('Program'),
       ),
+      bottomNavigationBar: BottomAppBar(
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          children: <Widget>[
+            Slider(
+              value: position,
+              onChangeStart: (_) {
+                seeking = true;
+              },
+              onChangeEnd: (pos) {
+                seeking = false;
+                backend.seekTo(pos);
+              },
+              onChanged: (pos) {
+                setState(() {
+                  position = pos;
+                });
+              },
+            ),
+          ],
+        ),
+      ),
     );
   }
+
+  @override
+  void dispose() {
+    super.dispose();
+    positionSubscription.cancel();
+  }
 }