Pull request for working encapsulated implementation of VoxelMouseAction.cpp/h

When clicking on a voxel, it is deemed 'selected'.
Then, while mouse is still downclicked, 

drag left: a voxel outline will appear where, by dragging further left, you may increase the size of the voxel to be created. Release to delete

Drag Right: Deletes the selected voxel upon release of click.

Drag Down: Moves towards the voxel upon release of click.