plugin_framework, flutter event handlers

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

flutter/lib/plugin/event.dart

@@ -0,0 +1,11 @@
+void handlePluginEvent(
+  Map<String, dynamic> evt,
+  String peer,
+  Function(Map<String, dynamic> e) handleMsgBox,
+) {
+  if (evt['content']?['c'] == null) return;
+  final t = evt['content']?['t'];
+  if (t == 'MsgBox') {
+    handleMsgBox(evt['content']?['c']);
+  }
+}