plugin_framework, flutter event handlers

Signed-off-by: fufesou <shuanglongchen@yeah.net>

flutter/lib/plugin/common.dart

@@ -0,0 +1,43 @@
+import 'dart:convert';
+
+typedef PluginId = String;
+
+// ui location
+const String kLocationHostMainDisplayOthers =
+    'host|main|settings|display|others';
+const String kLocationClientRemoteToolbarDisplay =
+    'client|remote|toolbar|display';
+
+class MsgFromUi {
+  String id;
+  String name;
+  String location;
+  String key;
+  String value;
+  String action;
+
+  MsgFromUi({
+    required this.id,
+    required this.name,
+    required this.location,
+    required this.key,
+    required this.value,
+    required this.action,
+  });
+
+  Map<String, dynamic> toJson() {
+    return <String, dynamic>{
+      'id': id,
+      'name': name,
+      'location': location,
+      'key': key,
+      'value': value,
+      'action': action,
+    };
+  }
+
+  @override
+  String toString() {
+    return jsonEncode(toJson());
+  }
+}