Pack icon path splicing function, improve the doc

[ggdream]

Hello, the main tasks are as follows:

1. Encapsulate the icon path splicing work into the library

   String path;
   if (Platform.isWindows) {
     path = p.joinAll([
       p.dirname(Platform.resolvedExecutable),
       'data/flutter_assets/assets',
       'app_icon.ico'
     ]);
   } else if (Platform.isMacOS) {
     path = p.joinAll(['AppIcon']);
   } else {
     path = p.joinAll([
       p.dirname(Platform.resolvedExecutable),
       'data/flutter_assets/assets',
       'app_icon.png'
     ]);
   }

2. Standardize the initialization parameter requirements, the title and icon are changed to mandatory parameters

   Future<bool> initSystemTray({
   required String title,
   required String iconPath,
   String? toolTip,
   })

3. In order to facilitate learning and use, a minimal example has been added

   Future<void> initSystemTray() async {
   final path =
     Platform.isWindows ? 'assets/app_icon.ico' : 'assets/app_icon.png';
   final menu = [
   MenuItem(label: 'Show', onClicked: _appWindow.show),
   MenuItem(label: 'Hide', onClicked: _appWindow.hide),
   MenuItem(label: 'Exit', onClicked: _appWindow.close),
   ];
   
   await _systemTray.initSystemTray(
   title: "system tray",
   iconPath: path,
   );
   
   await _systemTray.setContextMenu(menu);
   }