Added an isSoft field to the AttachmentData which is edited
by the Attachment Dialog Menu, sent over the network via
AvatarData identity packets and saved in the Interface.ini preferences.
AvatarData and AvatarBulkData version number has been bumped.
Changed Avatar attachment collections to use smart pointers to models
instead of raw ones. Removed _unusedAttachmentModels.
I don't think the caching was worth the added code complexity.
Is a subclass of Model, it overrides the updateClusterMatrices so it will pull
the actual joint matrices from a different rig override.
For the avatar soft attachment system, this override will be the Avatar::_skeletonModel rig.
This will give us the ability for an avatar to "wear" non-rigid attachments, such as clothing.
