Abstract batch gathering and processing code

I wanted to be able to run a Migration from a task in an install profile, and it would be nice if the code in migrate_ui.pages.inc had been abstract so I could re-use it. This is my first attempt. It's nice since I can start a batch just using the following:

$batch = new MigrateImportBatch();
$batch->addGroupMigration('my_migration_group');
$result = $batch->getBatch();
batch_set($result);