When actions were implemented, the only group of actions we needed were

menus. As we begin to use action for more purposes, we need other groups

of actions, meaning it's a good time to come up with a more general base

class for this.
This change introduces BaseGroupAction, which MenuAction is built

upon. This manages a group of actions, allowing for explicit ordering

and -- just like menus (though it would be up to an implementation how

it wants to handle this).
There's a JavaScript side as well, which is equivalent to what

MenuAction was before. MenuAction is now a very thin subclass over

GroupAction.
For testing, our formerly registry-specific set of test action classes

have been moved into a common module, along with some new actions for

groups.
On top of this, actions now have a parent registry, assigned when

registering and unassigned when unregistering. This is primarily for

unit test purposes, but removes the assumption of actions_registry for

child actions.

Unit tests pass.
Tested all the actions in the UI to make sure they still work and

still show the menu items in the right order.