Integration with webform

I've been doing some work on this but it's focused on submissions.

Here's some preliminary stuff. I've been able to insert submissions with text fields. I haven't gotten around to messing with more complex types.

Getting closer, I've tested most field types other than grid and hidden.

Realized we should pass the node into our DestinationHandlers. Also throw an exception if the pass in a non-webform node.

This is getting close. I'd love some maintainer feedback.

Huh, d.o seems to having some trouble, lost my previous reply - anyway, fields() can take $entity_type and $bundle parameters, so you can restrict it to $bundle=='webform'...

Meant to say:
I thought I was following the pattern in MigrateExtrasFlagHandler::fields() but now that I look at it they're calling the module_exists to see if it's safe to call the function. That code is commented out so I'll just delete it since I didn't actually use any of it.

This has a better __toString(), correctly computes update/insert counts when specifying sids an has some WIP code to pick the value of select components. Use the last patch then just drop this in over it. I didn't want to bother re-rolling right now.

I think this should be pretty good. Got the selects able to look up keys or values based on an argument.

Committed to 7.x-2.x