Squashed commit of the following:

commit 52b20d12a3593e776433c4f3ec1c6230755f1445
Author: Gunnar Sletta <gunnar.sletta@jollamobile.com>
Date:   Fri Mar 14 15:15:26 2014 +0100

    Fix potential crash during shutdown for QQuickWindows

    The cleanup() function would deregister the render loop from all
    windows the render loop had seen, but the render loop doesn't see
    windows until the window gets a showEvent and for some implementations
    it was removed as a result of hideEvent. So add explicit tracking to
    QSGRenderLoop which is managed by QQuickWindow's constructor and
    destructor. With this, we no longer need the lists from the
    subclasses, so these functions are removed again.

    Change-Id: I05e5507ad57e23c80bacd99752654cc7d0890dc1
    Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>

    Conflicts:
    	src/quick/items/qquickwindow.cpp

commit 60bec30befb6468db79ab2acac6fcc7a389acca8
Author: Gunnar Sletta <gunnar.sletta@jollamobile.com>
Date:   Wed Mar 5 15:38:02 2014 +0100

    Make sure QSGRenderLoop is cleaned up cleanly.

    e13547c595913c58e6bd6a5ed80fdc729fae7d47 used a global static to clean
    up QSGRenderLoop which is triggered very late, potentially after
    SG backend API plugins have been unloaded. This results in crashes
    when used in combination with scenegraph-playgrounds's customcontext.

    Partially revert the change and instead clean up at the time of
    QApp::aboutToQuit and make sure we also disconnect cleanly from
    all QQuickWindows.

    This change also ensures that QSGRenderLoop::windowDestroyed() gets
    called for all QQuickWindows registered with the render loop. This
    ensures that rendering stops and that scene graph nodes and resources
    will be cleaned up regardless of whether the application has
    remembered to delete the window or not. This is a good thing as it
    makes the scene graph shutdown a bit cleaner.

    Change-Id: I9cb9093979f8eac05542f118a6ff9cfe5c84f745
    Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>

    Conflicts:
    	src/quick/items/qquickwindow.cpp
    	src/quick/scenegraph/qsgrenderloop.cpp
    	src/quick/scenegraph/qsgrenderloop_p.h

Change-Id: I14b60be55ed5fde72f38f48a79edc0b49f853a0c
Merge pull request #6 from sletta/next

[qtdeclarative] Fix crash on shutdown