Enhance TP manifest documentation and add placeholder variable for paths.
Support for third_party_paths in Third Party Manifest.

Third party paths are those third party libs registered with the framework
that already exist on the system. Some projects like to check in third
party libs into version control (cough) or otherwise have them sitting on
their machine somewhere. With this feature, you can register third party
aliases in the manifest and use them in the framework without needing to
download them.