Switch QQuickWidgets to the software backend (2D renderer)

Now that https://bugreports.qt.io/browse/QTBUG-52909 is in place in qtdeclarative/dev (that will eventually become 5.8), applications can request the software backend by doing QQuickWindow::setSceneGraphBackend(QSGRendererInterface::Software) easly in main() before constructing any QQuickWindow/View/Widget.

Creator should investigate moving to this since it would avoid the runtime OpenGL dependency that causes so much trouble.

Some components, the profiler in particular, may have some issues with the migration since they may use custom Quick items in a way that is not necessarily compatible with the non-GL backends.

Performance is another question, for example when it comes to scrolling in the Welcome screen on large resolution screens. This has to be tested.