Added workable QScreen API on top of QPlatformScreen.

Added workable QScreen API on top of QPlatformScreen.

QPlatformIntegration::screens() no longer has to be implemented,
implementations should call QPlatformIntegration::screenAdded() for each
screen instead. This is for being able to support adding screens at
run-time later on, by connecting it to a signal in QGuiApplication.

The QGuiGLContext API has changed a bit, by not sending in all the
parameters in the constructor but instead having a create() function.
The createPlatformGLContext() factory in QPlatformIntegration takes a
QGuiGLContext * instead of a QSurfaceFormat and a share context, similar
to how the window and backing store factory functions work.

The XCB plugin has experimental support for connecting to multiple X
displays simultaneously, creating one or more QScreen for each.

Change-Id: I248a22a4fd3481280710110272c04a30a8021e8f
Reviewed-on: http://codereview.qt.nokia.com/2103
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Jørgen Lind <jorgen.lind@nokia.com>