Add basic inter-model mesh swapping capability

[shadowmage45]

E.g. FS / B9 mesh-switch. Uses meshes from transforms within a given model, enabling/disabling the meshes as specified in the config.

Single mesh-switch module responsible for managing a config-specified list of meshes. These meshes cannot be shared by other modules.

Should include basic attach-node positioning.

Needs to work correctly with the existing scaling functions.