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.

[mgerhardy]

There are several improvements now:

• 8b76f78445e316a3d8b69bd1c80bd22d12c2d8e8 convert the paint modifier into a brush
• 85234a78dbcb4753cc62f97cec89e1f91b78c0a5 and ad0d7962db602bb1c9c5e081d70c1cad41053364 added new paint modes
• 9992c283ed04014eafc79f92834ae31dae0a3be0 added a preview for the paint brush

but there are still task left to do:

• _plane mode should ignore aabb
• _center should work with _radius in combination with _single
• _radius should only go into one direction (see BrushContext::_cursorFace) (only paint the surface)