VOXEDIT: implement voxel selection algorithms

[mgerhardy]

Use the voxel::Voxel::_flags member (which is also available in the shader - see voxel::VoxelVertex) to perform voxel selection. Selected voxels should just get highlighted in the shader - not with yet another mesh.

[mgerhardy]

Select voxels by selecting colors from the palette widget

[mgerhardy]

Feedback about current situation "using selection tool is hard to see witch voxels selected, also didn't find a way to clear selection."

[mgerhardy]

Closing this in favor of #306