The 'cancelled' status is now tracked by a Future that's passed to
different asychronous tasks. That way it is possible to cancel all
running tasks before browsing another Pillar node.
We now draw the GUI using OpenGL in Python. This allows for much more
control on the Python side. It's still a prototype, and allows us to
test the features without depending on C support in Blender itself.
GUI code was taken from the Asset Flinger addon.
