Improve handling of XRandR events in XCB backend

Querying X server for data can be very expensive, especially when there
are multiple processes querying it at the same time (which is exactly what
happens when screen configuration changes and all Qt applications receive
XRandR change notifications). This patch is aiming to reduce the number of
queries to X server as much as possible by making use of detailed information
available in the RRCrtcChangeNotify and RROutputChangeNotify events.

Firstly, the backend now does not rebuild all QXcbScreens on any change (which
involved the very expensive xcb_randr_get_screen_resources() call), but only
builds the full set of QXcbScreens once in initializeScreens(), and then just
incrementally updates it.

Secondly, it avoids querying X server for all screens geometry as much as
possible, and only does so when CRTC/Output change notification for a particular
screen is delivered.

As a result, handling of all XRandR events on screen change is reduced from tens
of seconds to less then a seconds and applications are better responsive after
that, because we don't block the event loop for long. The X server is also more
responsive after the screen change, since we are not overloading it with requests.

Change-Id: I9b8308341cada71dfc9590030909b1e68a335a1f
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>