feat: add entry in left panel

flutter/lib/plugin/ui_manager.dart

@@ -0,0 +1,17 @@
+import 'package:flutter/material.dart';
+
+class PluginUiManager {
+  PluginUiManager._();
+
+  static PluginUiManager instance = PluginUiManager._();
+
+  Map<String, Widget> entries = <String, Widget>{};
+
+  void registerEntry(String key, Widget widget) {
+    entries[key] = widget;
+  }
+
+  void unregisterEntry(String key) {
+    entries.remove(key);
+  }
+}