Complete rewrite of threaded render loop.

This change starts using the superior implementation of the scene graph
render loop which has been worked on in the scenegraph-playground
project for a while. It uses a far more straightforward locking/sync
paradigm compared to the existing one and is less deadlock and error
prone. It also enables the scene graph thread to run on its own when
the GUI thread is blocked, enabling threaded animations.

This changes also introduces a naming change inside Qt Quick from
"Window Manager" -> "Render Loop" as that fits better to what the
code does.

Change-Id: I1c2170ee04fcbef79660bd7dae6cace647cdb276
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>