Add instruments selector

This also adds a tile to the temporary home screen for debugging
purposes.

lib/screens/home.dart

@@ -2,6 +2,7 @@ import 'package:flutter/material.dart';
 
 import '../backend.dart';
 import '../selectors/person.dart';
+import '../selectors/instruments.dart';
 
 class HomeScreen extends StatelessWidget {
   @override
@@ -32,6 +33,15 @@ class HomeScreen extends StatelessWidget {
                   fullscreenDialog: true,
                 )),
           ),
+          ListTile(
+            title: Text('Select instrument'),
+            onTap: () => Navigator.push(
+                context,
+                MaterialPageRoute(
+                  builder: (context) => InstrumentsSelector(),
+                  fullscreenDialog: true,
+                )),
+          ),
         ],
       ),
     );

lib/selectors/instruments.dart

@@ -0,0 +1,77 @@
+import 'package:flutter/material.dart';
+
+import '../backend.dart';
+import '../database.dart';
+import '../editors/instrument.dart';
+
+class InstrumentsSelector extends StatefulWidget {
+  @override
+  _InstrumentsSelectorState createState() => _InstrumentsSelectorState();
+}
+
+class _InstrumentsSelectorState extends State<InstrumentsSelector> {
+  Set<Instrument> selection = {};
+
+  @override
+  Widget build(BuildContext context) {
+    final backend = Backend.of(context);
+
+    return Scaffold(
+      appBar: AppBar(
+        title: Text('Select instruments'),
+        actions: <Widget>[
+          FlatButton(
+            child: Text('DONE'),
+            onPressed: () => Navigator.pop(context, selection.toList()),
+          ),
+        ],
+      ),
+      body: StreamBuilder(
+        stream: backend.db.allInstruments().watch(),
+        builder: (context, snapshot) {
+          if (snapshot.hasData) {
+            return ListView.builder(
+              itemCount: snapshot.data.length,
+              itemBuilder: (context, index) {
+                final instrument = snapshot.data[index];
+
+                return CheckboxListTile(
+                  title: Text(instrument.name),
+                  value: selection.contains(instrument),
+                  onChanged: (selected) {
+                    setState(() {
+                      if (selected) {
+                        selection.add(instrument);
+                      } else {
+                        selection.remove(instrument);
+                      }
+                    });
+                  },
+                );
+              },
+            );
+          } else {
+            return Container();
+          }
+        },
+      ),
+      floatingActionButton: FloatingActionButton(
+        child: const Icon(Icons.add),
+        onPressed: () async {
+          final Instrument instrument = await Navigator.push(
+              context,
+              MaterialPageRoute(
+                builder: (context) => InstrumentEditor(),
+                fullscreenDialog: true,
+              ));
+          
+          if (instrument != null) {
+            setState(() {
+              selection.add(instrument);
+            });
+          }
+        },
+      ),
+    );
+  }
+}