Refactor: Use material URIs when looking up linked definitions
Refactor|Material: Relocated material "isCustom" state to MaterialBind
Refactor: Renamed MaterialBind to MaterialManifest
MaterialManifest: Link associated material definitions earlier

It is no longer necessary to defer the linking of associated effect
definitions with MaterialManifests until after the derived material
is created. The effect defs can now be linked immediately following
the creation of the manifest.