Remove seperate role repesentation

The role and instrument tables have been merged into one (the instrument
table). There are not that many roles that aren't instruments and it is
much simpler to mange this way. The role editor and role selector have
been removed and the instrument related UI parts have been modified
accordingly.

lib/editors/instrument.dart

@@ -32,7 +32,7 @@ class _InstrumentEditorState extends State<InstrumentEditor> {
 
     return Scaffold(
       appBar: AppBar(
-        title: Text('Instrument'),
+        title: Text('Instrument/Role'),
         actions: <Widget>[
           FlatButton(
             child: Text('DONE'),

lib/editors/role.dart

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

lib/editors/work.dart

@@ -89,6 +89,7 @@ class WorkProperties extends StatelessWidget {
                 context,
                 MaterialPageRoute(
                   builder: (context) => InstrumentsSelector(
+                    multiple: true,
                     selection: instruments,
                   ),
                   fullscreenDialog: true,