Allow querying of the propagated light level.

This adds one Lua function, get_node_propagated_light, which works exactly
as get_node_light except that it excludes the entity at the specified location.

Class MapNode is extended to support this.

This allows extensions such as magic_lantern to work without removing the
object – each lamp would be removed prior to querying the light level then,
once the new setting had been determined, it would be re-created.
Unfortunately, this caused noticeable flicker...

The problem? The scene may be rendered while the lamp is removed.