Add initial selection to instrument selector

Also use this from the work editor.

lib/editors/work.dart

@@ -119,7 +119,9 @@ class _WorkEditorState extends State<WorkEditor> {
               final List<Instrument> selection = await Navigator.push(
                   context,
                   MaterialPageRoute(
-                    builder: (context) => InstrumentsSelector(),
+                    builder: (context) => InstrumentsSelector(
+                      selection: instruments,
+                    ),
                     fullscreenDialog: true,
                   ));
 

lib/selectors/instruments.dart

@@ -5,6 +5,12 @@ import '../database.dart';
 import '../editors/instrument.dart';
 
 class InstrumentsSelector extends StatefulWidget {
+  final List<Instrument> selection;
+
+  InstrumentsSelector({
+    this.selection,
+  });
+
   @override
   _InstrumentsSelectorState createState() => _InstrumentsSelectorState();
 }
@@ -12,6 +18,15 @@ class InstrumentsSelector extends StatefulWidget {
 class _InstrumentsSelectorState extends State<InstrumentsSelector> {
   Set<Instrument> selection = {};
 
+  @override
+  void initState() {
+    super.initState();
+
+    if (widget.selection != null) {
+      selection = widget.selection.toSet();
+    }
+  }
+
   @override
   Widget build(BuildContext context) {
     final backend = Backend.of(context);
@@ -65,7 +80,7 @@ class _InstrumentsSelectorState extends State<InstrumentsSelector> {
                 builder: (context) => InstrumentEditor(),
                 fullscreenDialog: true,
               ));
-          
+
           if (instrument != null) {
             setState(() {
               selection.add(instrument);