Class AddToCampaignHandler - handle the AddToCampaign action.

This is a class designed to be delegated to by a Form action handler method in the EditForm of a LeftAndMain
child class.

Add To Campaign can be seen as an item action like "publish" or "rollback", but unlike those actions
it needs one additional piece of information to execute, the ChangeSet ID.

So this handler does one of two things to respond to the action request, depending on whether the ChangeSet ID
was included in the submitted data
- If it was, perform the Add To Campaign action (as per any other action)
- If it wasn't, return a form to get the ChangeSet ID and then repeat this action submission

To use, you'd add an action to your LeftAndMain subclass, like this:

function addtocampaign($data, $form) {
$handler = AddToCampaignHandler::create($form, $data);
return $handler->handle();

and add an AddToCampaignHandler_FormAction to the EditForm, possibly through getCMSActions