Support namespaced queue classes and queues not based on the `queue` table

I needed to create a queue similar to SystemQueue that operates on a different table. The queue class has a full namespace \Drupal\my_module\Queue\SystemSetQueue.

Queue UI makes several assumptions that broke under those circumstances:

• Assumes that prepending 'QueueUI' to a class name will be a valid class name. That is not true for namespaced class names.
• Assumes that all queue names are listed in the queue table under the name column. That is not true for queues that don't operate using the database or use a different table.

The following patch will address these issue by making use of the existing (and not used) hook_queue_info. This will allow other modules to expose their queue names. It also includes some other minor misc issues I've been noticing.

Thanks for maintaining this module!