[CONTENT] Create collapsible families of extensions

[gynt]

Proposal Modify definition.yml to include a family: field that can list the family of extensions it belongs to. This way, we can collapse extensions of the same family into one display node. The family needs to define a root extension that will be the one for display purposes. And if nothing is uncollapsed then this root is activated upon click.

Root extension:

name: extension-A
version: B
family:
- name: extension-A-family
  root: true

Family extension:

name: extension-A-files
version: B
family:
- name: extension-A-family