Add LibEventCpp library

[nguyenchiemminhvu]

LibEventCpp is a lightweight and portable C++14 library for event-driven
programming. Implemented in a single header file for easy integration.

Features:

• Message event handler: Asynchronous message queue with event looper
• Signals and slots: Type-safe callback connections (Qt-style)
• Time events: POSIX timers with callback support
• Once events: Execute callbacks conditionally (once per life, N times,
  value change, interval)
• Toggle events: One-shot triggers with reset capability
• File descriptor events: Monitor file descriptors using poll()
• Signal events: POSIX signal handling wrapper
• File system events: Monitor file system changes using inotify (Linux)