Implement "headless mode" for hidden QQuickCanvases

When all views are hidden, we stop the rendering thread,
kill the OpenGL context and all scene graph content.
The entire scenegraph is recreated based on the QML scene
when a view is shown again.

Change-Id: I734619d9f29263a5cdecbcc9b88c3808d1d64a7f
Reviewed-by: Kim M. Kalland <kim.kalland@nokia.com>