Updated base node class to support several outputs.
Now bricks support 2 outputs : greyscale depth map, and (random) brick color.
Now Voronoi supports 3 outputs : center distance, edge distance and cell color.
Updated bricks example to use brick color output.
now load/save dialog use the non-editor classes (thus usable when running the project).
Refactored graphedit and node code (mainly load/save and adding and removing nodes).
