feat: add icon support for Linux desktop autostart file

merged upstream pr leanflutter#40

Author: EyKettle

lib/src/app_auto_launcher_impl_linux.dart

@@ -26,6 +26,7 @@ class AppAutoLauncherImplLinux extends AppAutoLauncher {
 [Desktop Entry]
 Type=Application
 Name=$appName
+Icon=$appName
 Comment=$appName startup script
 Exec=${args.isEmpty ? appPath : '$appPath ${args.join(' ')}'}
 StartupNotify=false