mobile: Update to new client API

The settings screen was refactored too.

mobile/lib/widgets/works_by_composer.dart

@@ -1,39 +0,0 @@
-import 'package:flutter/material.dart';
-import 'package:musicus_database/musicus_database.dart';
-
-import '../backend.dart';
-
-class WorksByComposer extends StatelessWidget {
-  final int personId;
-  final void Function(Work work) onTap;
-
-  WorksByComposer({
-    this.personId,
-    this.onTap,
-  });
-
-  @override
-  Widget build(BuildContext context) {
-    final backend = Backend.of(context);
-
-    return StreamBuilder<List<Work>>(
-      stream: backend.db.worksByComposer(personId).watch(),
-      builder: (context, snapshot) {
-        if (snapshot.hasData) {
-          return ListView.builder(
-            itemCount: snapshot.data.length,
-            itemBuilder: (context, index) {
-              final work = snapshot.data[index];
-              return ListTile(
-                title: Text(work.title),
-                onTap: () => onTap(work),
-              );
-            },
-          );
-        } else {
-          return Container();
-        }
-      },
-    );
-  }
-}