Add basic work selector

lib/selectors/work.dart

@@ -0,0 +1,71 @@
+import 'package:flutter/material.dart';
+
+import '../backend.dart';
+import '../database.dart';
+import '../editors/work.dart';
+
+// TODO: Lazy load works and/or optimize queries.
+class WorkSelector extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    final backend = Backend.of(context);
+
+    return Scaffold(
+      appBar: AppBar(
+        title: Text('Select work'),
+      ),
+      body: StreamBuilder<List<Person>>(
+        stream: backend.db.allPersons().watch(),
+        builder: (context, snapshot) {
+          if (snapshot.hasData) {
+            return ListView.builder(
+              itemCount: snapshot.data.length,
+              itemBuilder: (context, index) {
+                final person = snapshot.data[index];
+                final title = Text('${person.lastName}, ${person.firstName}');
+                return StreamBuilder<List<Work>>(
+                  stream: backend.db.worksByComposer(person.id).watch(),
+                  builder: (context, snapshot) {
+                    if (snapshot.hasData && snapshot.data.isNotEmpty) {
+                      return ExpansionTile(
+                        title: title,
+                        children: <Widget>[
+                          for (final work in snapshot.data)
+                            ListTile(
+                              title: Text(work.title),
+                              onTap: () => Navigator.pop(context, work),
+                            ),
+                        ],
+                      );
+                    } else {
+                      return ListTile(
+                        title: title,
+                      );
+                    }
+                  },
+                );
+              },
+            );
+          } else {
+            return Container();
+          }
+        },
+      ),
+      floatingActionButton: FloatingActionButton(
+        child: const Icon(Icons.add),
+        onPressed: () async {
+          final Work work = await Navigator.push(
+              context,
+              MaterialPageRoute(
+                builder: (context) => WorkEditor(),
+                fullscreenDialog: true,
+              ));
+
+          if (work != null) {
+            Navigator.pop(context, work);
+          }
+        },
+      ),
+    );
+  }
+}