Add music library path setting

This introduces a new state of the backend called "setup". If the
music library path is not set, the backend goes into that state and the
app widget can show the UI to set it up. This also introduces a new
dependency on shared_preferences.

lib/backend.dart

@@ -1,12 +1,14 @@
 import 'package:flutter/widgets.dart';
 import 'package:permission_handler/permission_handler.dart';
 import 'package:rxdart/rxdart.dart';
+import 'package:shared_preferences/shared_preferences.dart';
 
 import 'database.dart';
 
 enum BackendStatus {
   loading,
   needsPermissions,
+  setup,
   ready,
 }
 
@@ -33,6 +35,9 @@ class BackendState extends State<Backend> {
 
   Database db;
   BackendStatus status = BackendStatus.loading;
+  String musicLibraryPath;
+
+  SharedPreferences _shPref;
 
   @override
   void initState() {
@@ -49,6 +54,9 @@ class BackendState extends State<Backend> {
   }
 
   Future<void> _load() async {
+    _shPref = await SharedPreferences.getInstance();
+    musicLibraryPath = _shPref.getString('musicLibraryPath');
+
     db = Database('musicus.sqlite');
 
     final permissionStatus =
@@ -58,6 +66,16 @@ class BackendState extends State<Backend> {
       setState(() {
         status = BackendStatus.needsPermissions;
       });
+    } else {
+      await _loadMusicLibrary();
+    }
+  }
+
+  Future<void> _loadMusicLibrary() async {
+    if (musicLibraryPath == null) {
+      setState(() {
+        status = BackendStatus.setup;
+      });
     } else {
       setState(() {
         status = BackendStatus.ready;
@@ -68,7 +86,7 @@ class BackendState extends State<Backend> {
   Future<void> requestPermissions() async {
     final result =
         await _permissionHandler.requestPermissions([PermissionGroup.storage]);
-    
+
     if (result[PermissionGroup.storage] == PermissionStatus.granted) {
       setState(() {
         status = BackendStatus.ready;
@@ -78,6 +96,15 @@ class BackendState extends State<Backend> {
 
   Future<void> openAppSettings() => _permissionHandler.openAppSettings();
 
+  Future<void> setMusicLibraryPath(String path) async {
+    musicLibraryPath = path;
+    await _shPref.setString('musicLibraryPath', path);
+    setState(() {
+      status = BackendStatus.loading;
+    });
+    await _loadMusicLibrary();
+  }
+
   void startPlayer() {
     playerActive.add(true);
   }