Add access to person editor to person selector

The person editor can be reached via the FAB. If a new person is
created, it will be selected.

lib/selectors/person.dart

@@ -1,6 +1,8 @@
 import 'package:flutter/material.dart';
 
 import '../backend.dart';
+import '../database.dart';
+import '../editors/person.dart';
 
 class PersonsSelector extends StatelessWidget {
   @override
@@ -29,7 +31,23 @@ class PersonsSelector extends StatelessWidget {
           } else {
             return Container();
           }
-          }),
+        },
+      ),
+      floatingActionButton: FloatingActionButton(
+        child: const Icon(Icons.add),
+        onPressed: () async {
+          final Person person = await Navigator.push(
+              context,
+              MaterialPageRoute(
+                builder: (context) => PersonEditor(),
+                fullscreenDialog: true,
+              ));
+
+          if (person != null) {
+            Navigator.pop(context, person);
+          }
+        },
+      ),
     );
   }
 }