Complete the attachment framework.

The child ID can now be checked against the parent ID in content_cao.cpp so the parent can be detected. Actual attachment code to come

Divide attachment system between server attachments and client attachments, neither coded right now. As explained in the code comment:

// Attachments need to be handled on both the server and client.
// If we attach only on the server, models (which are client-side)
// can't be read so we don't know the origin and orientation of bones.
// If we attach only on the client, the real position of attachments is
// not updated and you can't click them for example.