add peekEventQueue API

This will make it simpler to port Qt4/Xlib based
applications to Qt5 when Qt4 based application
uses Xlib's event handling APIs directly such as:
XNextEvent+XPutBackEvent and XPeekEvent. It
is not possible to use those with Qt5 because
we set XCB to be the owner of X event queue with:

XSetEventQueueOwner(dpy, XCBOwnsEventQueue);

Which means that only xcb event handling functions
can be used to read events from X server. XCB does
not have equivalents for the above mentioned Xlib APIs.
peekEventQueue() adds this behavior to QX11Info.