Move XCB event reading to a separate thread.

Move XCB event reading to a separate thread.

Work-around for bug in XCB which causes a xcb_wait_for_reply to block
if xcb_poll_for_events() is called simultaneously from a different
thread. If the XCB version is too old this work-around causes even
more problems, so we kill two birds with one stone by only using the
work-around if the XCB version has the recent
xcb_poll_for_queue_event() function, which we also need to read
events from a separate thread with reasonable efficiency.

Change-Id: I8a899dad6ded381ce42cba0112e77da3c8aa6887
Reviewed-on: http://codereview.qt.nokia.com/3612
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>