Add ensemble editor

lib/editors/ensemble.dart

@@ -0,0 +1,66 @@
+import 'package:flutter/material.dart';
+
+import '../backend.dart';
+import '../database.dart';
+
+class EnsembleEditor extends StatefulWidget {
+  final Ensemble ensemble;
+
+  EnsembleEditor({
+    this.ensemble,
+  });
+
+  @override
+  _EnsembleEditorState createState() => _EnsembleEditorState();
+}
+
+class _EnsembleEditorState extends State<EnsembleEditor> {
+  final nameController = TextEditingController();
+
+  @override
+  void initState() {
+    super.initState();
+
+    if (widget.ensemble != null) {
+      nameController.text = widget.ensemble.name;
+    }
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final backend = Backend.of(context);
+
+    return Scaffold(
+      appBar: AppBar(
+        title: Text('Ensemble'),
+        actions: <Widget>[
+          FlatButton(
+            child: Text('DONE'),
+            onPressed: () async {
+              final ensemble = Ensemble(
+                id: widget.ensemble?.id ?? generateId(),
+                name: nameController.text,
+              );
+
+              await backend.db.updateEnsemble(ensemble);
+              Navigator.pop(context, ensemble);
+            },
+          )
+        ],
+      ),
+      body: ListView(
+        children: <Widget>[
+          Padding(
+            padding: const EdgeInsets.all(16.0),
+            child: TextField(
+              controller: nameController,
+              decoration: InputDecoration(
+                labelText: 'Name',
+              ),
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+}