* Allow reloading groups without restart
* Test to make sure automation listeners are removed.
* Remove unused imports for group tests
* Simplify group config validation
* Add prepare_reload function to entity component
* Migrate group to use entity_component.prepare_reload
* Migrate automation to use entity_component.prepare_reload
* Clean up group.get_entity_ids
* Use cv.boolean for group config validation
* Accept group without entities in configuration.
People seem to use these as placeholders for future expansion of their
home automation dreams, and we used to accept them. We still have to
specify at least one of 'name', 'view' or 'icon' so that the group is
parsed as a dictionary.
* Also accept empty entities: key in a group.
* Additional fix for empty entities value in a group config.
