fix: raw display should release file descriptors (issue #221)
fix: raw display should release file descriptors (issue #221)

`raw_display.Screen.hook_event_loop` does not fill
`self._current_event_loop_handles` with handled file descriptors.
So, `raw_display.Screen.hook_event_loop` can not release then.

wardi/urwid,urwid/urwid,rndusr/urwid,rndusr/urwid,wardi/urwid,inducer/urwid,tonycpsu/urwid,rndusr/urwid,wardi/urwid,inducer/urwid,tonycpsu/urwid,inducer/urwid,tonycpsu/urwid,urwid/urwid,urwid/urwid