Flush xcb connection before EventDispatcher is going to block

The non-threaded QXcbEventReader invokes processXcbEvents when the
EventDispatcher is about to block. This method ensures that the xcb
connection is going to flush. Applications can use low level xcb code
in that case without having to ensure to flush the connection before
going to block again.

With the threaded QXcbEventReader this didn't work and applications
which for example changed a window property and waited for the matching
property notify event were stalled.

This change ensures that also in the threaded case the connection gets
flushed when the EventDispatcher is going to block.

Change-Id: If1dc5eb96e2f1bde10b7a40af550b0608c62f70c
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>