Add Helix.Core.Listener (global object listener)

[renatomassaro]

Closes #268.

• [x] Interface, handlers, models
• [x] Test
• [x] Doc
• [x] Types

---

This change is 

[sourcelevel-bot[bot]]

Ebert has finished reviewing this Pull Request and has found:

• 1 possible new issue (including those that may have been commented here).

You can see more details about this review at https://ebertapp.io/github/HackerExperience/Helix/pulls/317.

[renatomassaro]

Reviewed 25 of 30 files at r1, 5 of 5 files at r2. Review status: all files reviewed at latest revision, all discussions resolved.

---

Comments from Reviewable