Refactor Simplistic_Editor (#1375)

* refactor out FormattingToolbar and TextEditingDeltaHistoryView from main`

* replace togglebuttonstatemanager and texteditingdeltahistorymanager with appstatemanager

* fix up tests

* clean up

* Add ReplacementTextEditingController to AppState

* formatting

* `dart format`

Co-authored-by: Renzo Olivares <roliv@google.com>
Co-authored-by: Brett Morgan <brettmorgan@google.com>

simplistic_editor/lib/formatting_toolbar.dart

@@ -0,0 +1,47 @@
+import 'package:flutter/material.dart';
+
+import 'app_state.dart';
+import 'app_state_manager.dart';
+
+/// The toggle buttons that can be selected.
+enum ToggleButtonsState {
+  bold,
+  italic,
+  underline,
+}
+
+class FormattingToolbar extends StatelessWidget {
+  const FormattingToolbar({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    final AppStateManager manager = AppStateManager.of(context);
+
+    return Padding(
+      padding: const EdgeInsets.symmetric(vertical: 8.0),
+      child: Row(
+        mainAxisAlignment: MainAxisAlignment.center,
+        children: [
+          ToggleButtons(
+            borderRadius: const BorderRadius.all(Radius.circular(4.0)),
+            isSelected: [
+              manager.appState.toggleButtonsState
+                  .contains(ToggleButtonsState.bold),
+              manager.appState.toggleButtonsState
+                  .contains(ToggleButtonsState.italic),
+              manager.appState.toggleButtonsState
+                  .contains(ToggleButtonsState.underline),
+            ],
+            onPressed: (index) => AppStateWidget.of(context)
+                .updateToggleButtonsStateOnButtonPressed(index),
+            children: const [
+              Icon(Icons.format_bold),
+              Icon(Icons.format_italic),
+              Icon(Icons.format_underline),
+            ],
+          ),
+        ],
+      ),
+    );
+  }
+}