ui: Implement track object model API

- Introduce the Track/Group/Workspace object model API.
  - This allows nested groups (including headless groups) and paves the
    way for workspaces.
- Use it for creating tracks, replacing the current state model.
- Rename trackKey -> trackUri everywhere to make it clear that URIs
  are being used, as the new track nodes have no key value any more
  and are purely identified using their URI.
- Add a the group name prefixed to the front of pinned tracks to add
  some context to pinned tracks.
- All thread tracks are now contained within headless groups within
  their parent process tracks.

Change-Id: I73c59f0eef3075c8d6bbceab5b7eb024315b5122