Automatic library loading for unknown typeIds

[davidsminor]

We often run into situations where, for example, the user imports a python module, writes out a scene cache containing an object defined in one of that python module's libraries, and is henceforth unable to read that scene cache because the data file contains an unregistered typeId (unless they remember to import the python module so the library gets loaded or whatever).

This can get annoying, so it'd be good to have some kind of mechanism for automatically loading libraries for unregistered type ids