Implement Entity Attachments

[zackthehuman]

It should be possible to attach one Entity to another, calculating its position relative to the attached-to object. This kind of thing is needed for moving platforms, rush jet, shield weapons, etc.

Possible implementation could be via an Attachment object, which all Entities have a unique_ptr to. When it's non-null, positions are calculated based on the attachment. When it is null, positions are calculated in absolute terms.