Add ability to save voxel selection as 3d brush

[darkhog]

Basically think brushes in 2d drawing apps, like DeluxePaint, Photoshop, GIMP, etc. but 3d and made of voxels instead of pixels. This would make Goxel even more powerful and quick to make voxel art with.

There should be two kinds of voxel brushes you could make:

• one with no preserved color information, where when painting with the brush the voxels being created would be colored with the current color
• one with preserved color information, here the current color would be ignored and instead the voxels would keep the colors from the brush.

[guillaumechereau]

Yes that would be nice indeed.