Add role selector

lib/selectors/role.dart

@@ -0,0 +1,53 @@
+import 'package:flutter/material.dart';
+
+import '../backend.dart';
+import '../database.dart';
+import '../editors/role.dart';
+
+class RolesSelector extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    final backend = Backend.of(context);
+
+    return Scaffold(
+      appBar: AppBar(
+        title: Text('Select role'),
+      ),
+      body: StreamBuilder<List<Role>>(
+        stream: backend.db.allRoles().watch(),
+        builder: (context, snapshot) {
+          if (snapshot.hasData) {
+            return ListView.builder(
+              itemCount: snapshot.data.length,
+              itemBuilder: (context, index) {
+                final role = snapshot.data[index];
+
+                return ListTile(
+                  title: Text(role.name),
+                  onTap: () => Navigator.pop(context, role),
+                );
+              },
+            );
+          } else {
+            return Container();
+          }
+        },
+      ),
+      floatingActionButton: FloatingActionButton(
+        child: const Icon(Icons.add),
+        onPressed: () async {
+          final Role role = await Navigator.push(
+              context,
+              MaterialPageRoute(
+                builder: (context) => RoleEditor(),
+                fullscreenDialog: true,
+              ));
+
+          if (role != null) {
+            Navigator.pop(context, role);
+          }
+        },
+      ),
+    );
+  }
+}