Entities which has the property of solid=0 are non-solid.
They won't collide with solid entities, nor with other non-solids entities.
The only thing they will collide with is the terrain.
This property tells how the entity should behave when put into
different modes. Currently we only allow for specific transformations
to occur.
The mode specifications are registed in properties where "mode-" forms
a prefix, and the mode a suffix. For example, "mode-planted".
This also does away with the "plantable" property.
When checking visibility, we now use the Domains more extensively.
The base rules are:
* If an entity is a child of a domain-less entity, visiblity for the
child is the same as for the parent.
* If an entity is a child of a domain-holding entity, visibility is
determined by the Domain.
* Entities cannot observe things above a Domain.
* Entities can observe things below a Domain, if the Domain allows it.
These rules are not yet set in stone. We need some more time figuring out
exactly how we want visibility implemented. We're striving for a balance
between a powerful but not too complex system, with good performance.
