Removed unneeded comments.
Replace RenderSettings with a plain dictionary.

Working with dictionaries in Python is way better than trying to invent
my own storage container.
Visibility of slices in multiRender is now also saved to project.
Save the opacity of the volumes to the project.
Interaction performance improvement.

Fixed how volume properties are updated. Instead of removing and adding
entire volumes, now it only updates the volume properties. This speeds
up interaction with the parameter widgets.
Added a hue slider to the simple volume property visualization.
Relocate some classes and save camera position and rotation.