views integration for map tables

Here's a work in progress patch.

It works, and adds:

- a relationship from entity base tables to map tables for any migrations into that entity
- a field for the source id

Needs work:

- This just shows the first sourceID for now, and assumes it's numeric. Needs work to expose all source id fields using the appropriate handler
- MigrateDestinationEntity somewhat messily needs to be told the map table. I couldn't see how to get the name of the map table from within the destination class.
- similarly, MigrateDestinationEntity needs to get the migration details to show the description in the Views data help text.

> hook functions named hook_views_data_alter() - how would the hooks be invoked?

The hook functions named hook_views_data_alter() are in the classes, and get called by the actual hook_views_data_alter().... which I seemed to have missed off in my patch!!

(This is a standard pattern for Views, btw: views handler classes have methods like hook_menu() to return data for views_menu() for the handler.)

> I think the destination handler is the wrong place to implement this - it should be the map classes

So the chain would go:

- actual hook implementation
-- map class
--- destination handler

Seems fair enough to me, though I could do with hints on how to reach one class from the other, such as the migration class.

Did a search to see if anyone had worked on this, and what do you know, I did, 3 years ago! :)

And it turns out the file I missed off in the patch from 3 years ago is sitting on my hard drive because I hadn't got round to deleting the project it was written for :)

Here's a complete patch.

Just tested it with a node migration -- works great. I can build a view of nodes and add the relationship to the map table, set it to 'required' and I've got an audit view :)

Argh. Forgot to roll relative.