Add support for moving a Note to another folder

The Folder selection dialog needs a lot of work, but it's a start.

lib/widgets/folder_selection_dialog.dart

@@ -0,0 +1,31 @@
+import 'package:flutter/material.dart';
+import 'package:gitjournal/core/notes_folder.dart';
+import 'package:gitjournal/widgets/folder_tree_view.dart';
+import 'package:provider/provider.dart';
+
+typedef NoteFolderCallback = void Function(NotesFolder);
+
+class FolderSelectionDialog extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    final notesFolder = Provider.of<NotesFolder>(context);
+
+    var body = Container(
+      width: double.maxFinite,
+      child: FolderTreeView(
+        rootFolder: notesFolder,
+        onFolderEntered: (NotesFolder destFolder) {
+          Navigator.of(context).pop(destFolder);
+        },
+        longPressAllowed: false,
+      ),
+    );
+
+    return AlertDialog(
+      title: const Text('Select a Folder'),
+      content: body,
+    );
+  }
+}
+
+// FIXME: Add the previously as a radio button selected Folder