Implement proper selection of volume in editor mode

[David Williams]

Selection is currently implemented with a single box collider around the volume. This means that is can be selected even by clicking an empty voxel (which feels strange). Worse, it makes it hard to select objects inside the volume (but above the ground) as the volume tends to get selected instead.

[David Williams]

This will have to wait until a later release.

[David Williams]

This is a feature request and has been moved to Trello: https://trello.com/c/0BJmpjqh/10-improve-volume-selection