Refactor.
Increase the update rate of the render widgets.

This makes sure that the visualization will not be distorted too much
during interaction.
Improve the coloring of the landmarks.
Make sure that the landmark is shown as soon as it is added / changed.
Make it possible to set landmarks also with the space bar.
Refactor.
Update the previous transfer functions instead of creating new transfer functions every time.
Make sure that when an empty / no volume is loaded, that it won't show up in the render.
Increase interaction speed of render widgets even more.
Don't set the text for non-existing labels…