This includes using `''` instead of `None` in some cases where an empty
string conveys 'nothing' equally well as `None`; in such cases keeping the
type the same rather than switching to another type is preferred.
The biggest issue was the construction of an asyncio.Semaphore() while the
default loop is alive, and then creating a new loop on win32.
I've also taken the opportunity to explicitly pass our loop to some calls,
rather than expecting them to use the correct one automagically, and added
some more explicit timeout handling to the semaphore usage.
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.
