Add app-menu to the Productivity

[barseghyanartur]

Added app-menu, the missing Applications Menu for macOS, to the Productivity section.

https://github.com/barseghyanartur/app-menu/

0. I have read the Contribution Guidelines
1. Project URL:
2. Why should this project be added: It's a useful add-on, that I as a Linux user missed on macOS.
3. End all descriptions with a full stop/period
4. Entry is ordered alphabetically
5. Appropriate icon(s) added if applicable (OSS, freeware)