This PR shows an attempt at the 3d renderer that supports an arbitrary number of channels, each with their own LUT selector.

However, blending is not working correctly. The problem is that, when setting the mode to translucent, the displayed volume is completely dark.

The UI layout is correct, and the colormap selector from the LUT item works.

The APIs are also as I would like to have them, where renderer.set_volumes accepts a dictionary of {channel_name: volume_data}. Note that the run method of the renderer will set up the acdc QApplication if missing.

For example, one can do this

from cellacdc.renderer3d import VolumeRenderer3DWindow

mkate_data = skimage.io.imread(mkate_filepath)
volumes = {
    'mKate': mneon_data,
}

renderer = VolumeRenderer3DWindow()
renderer.set_volumes(
    volumes,
)
renderer.run()