Refine event handling

[jserv]

The internal event handling relied on the poll(2) for monitoring X11 events via I/O multiplexing. However, for backends such as SDL, this manipulation causes problems since the internal file descriptors associated with underlying system events are hidden and not exposed. Thus, we should avoid polling the file descriptors. Instead, let the backend handle the event loop when possible. See commit e935b0d .

For #3, we may provide poll(2) based mechanism to monitor events when requested.

[shengwen-tw]

Reference for implementing Coroutine in C:
https://github.com/shengwen-tw/coroutine_c