When actions were first introduced, an action could specify an

attachment point that they wanted to render into. These were strings

that corresponded to a template tag call, rendering all actions attached

to that point at that location.
This change builds upon that concept by introducing a formal

registration of attachment points. These provide a level of control for

the rendering and state of any actions placed within them, allowing, for

example, one attachment point to render actions as a list of links and

another to render those same actions as a list of buttons.
Each attachment point has a default renderer that's used if the action

being rendered doesn't specify its own default. The intended approach

going forward is to have actions only specify a renderer if they have a

strong need for certain rendering behavior, but to otherwise leave that

up to the parent action group, attachment point, or to the (coming soon)

placement configuration.
Attachment points may also specify a list of actions to directly

include, in the order included. These may be actions registered to this

attachment point, or they may not be. This allows an attachment point to

easily include a set of actions rendered under the attachment point's

control for special extension use cases.

Unit tests pass.
Tested all current actions in the UI. They all appear and behave as

expected.
Tested with in-progress changes that move Quick Access actions to this

new system, and verified they rendered as expected.