mobile: Integrate with server

mobile/lib/selectors/ensemble.dart

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+import 'package:flutter/material.dart';
+import 'package:musicus_database/musicus_database.dart';
+
+import '../editors/ensemble.dart';
+import '../widgets/lists.dart';
+
+/// A screen to select an ensemble.
+///
+/// If the user has selected one, it will be returned as an [Ensemble] object
+/// using the navigator.
+class EnsembleSelector extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: Text('Select ensemble'),
+      ),
+      body: EnsemblesList(
+        onSelected: (ensemble) {
+          Navigator.pop(context, ensemble);
+        },
+      ),
+      floatingActionButton: FloatingActionButton(
+        child: const Icon(Icons.add),
+        onPressed: () async {
+          final Ensemble ensemble = await Navigator.push(
+            context,
+            MaterialPageRoute(
+              builder: (context) => EnsembleEditor(),
+              fullscreenDialog: true,
+            ),
+          );
+
+          if (ensemble != null) {
+            Navigator.pop(context, ensemble);
+          }
+        },
+      ),
+    );
+  }
+}