feat: support max running tasks config (#112)

ui/flutter/assets/locales/en_US.json

@@ -24,6 +24,7 @@
   "downloadDir": "Download Directory",
   "downloadDirValid": "Please select the download directory",
   "connections": "Connections",
+  "maxRunning": "Max Running Tasks",
   "items": "@count items",
   "subscribeTracker": "Subscribe Tracker",
   "subscribeFail": "Subscribe failed, please check network or try again later",

ui/flutter/assets/locales/ru_RU.json

@@ -24,6 +24,7 @@
   "downloadDir": "Папка загрузки",
   "downloadDirValid": "Пожалуйста, выберите папку для загрузки",
   "connections": "Соединений",
+  "maxRunning": "Максимальное количество активных задач",
   "items": "@подсчет элементов",
   "subscribeTracker": "Подпись трекера",
   "subscribeFail": "Подписка не удалась, пожалуйста, проверьте сеть или повторите попытку позже",

ui/flutter/assets/locales/zh_CN.json

@@ -24,6 +24,7 @@
   "downloadDir": "下载目录",
   "downloadDirValid": "请选择下载目录",
   "connections": "连接数",
+  "maxRunning": "最大下载数",
   "items": "@count 条",
   "subscribeTracker": "订阅 Tracker",
   "subscribeFail": "订阅失败，请检查网络或稍后重试",