Attachment points and groups both support specifying default renderers

for any actions nested within. This applied to both standard actions and

group actions, which would do the wrong thing for group actions, and

necessitated that group actions always specify their own renderers.
These can now specify the default renderer for any group actions. With

this, an attachment point for a menubar could specify that all group

actions are rendered as menus, and groups in sidebars are rendered as

subsections, as a couple of examples.
As an internal optimization, actions now have an is_action_group

boolean property, which differentiates between an action and an action

group. This avoids the more costly isinstance checks that'd otherwise

need to be done multiple times per action render.
I also noted cases where fallback renderers weren't being used, and

where an detailed menu item action failed to render correctly with an

action lacking an icon. These have been fixed.

Unit tests pass.
Tested that existing actions work as expected.
Tested along with the upcoming addition of sidebar renderers.