LPS-48733 New registry for registering XStream aliases

Hey Brian,

Miguel has removed the possibility for setting aliases during export and import due to modularity concerns, and I was planning to add it back to fit the modular approach.

I've created a registry which makes it possible to register aliases for entities and this will be used to set up XStream during the export/import process.

This pull request contains the registry and some additional code, but I've only applied this to the Bookmarks so far as an example, I'll send the rest later, in a separate pull request.

This was reviewed by Miguel as well here: https://github.com/migue/liferay-portal/pull/284

Thanks,

Máté