VOXEDIT: paint modifier should be able to paint with gradients

[mgerhardy]

by clicking a voxel it should be possible to define a gradient or a mask to also colorize neighbored voxels

currently you can define a radius only to paint bigger areas around the cursor - but all in the same color. It would be nice to get stuff like darken or brighten the colors - and so to apply the above mentioned gradient on those actions.